|Region||Ireland, Isle of Man, western coast of Great Britain|
|Era||6th century–10th century; evolved into Middle Irish about the 10th century|
Old Irish (sometimes called Old Gaelic) is the name given to the oldest form of the Goidelic languages for which extensive written texts are extant. It was used from c. 600–c. 900 AD. The primary contemporary texts are dated c. 700 – 850 AD; by 900 AD the language had already transitioned into early Middle Irish. Some Old Irish texts date from 10th century, although these are presumably copies of texts composed at an earlier time period. Old Irish is thus the ancestor of Modern Irish, Scottish Gaelic, and Manx. Broadly speaking, the grammar and sound systems of the modern languages are simpler than those of Old Irish.
Old Irish is well-known for its highly complex system of allomorphy, especially in verbal conjugations. Compare asberat "they say" vs. ní-epret "they do not say" or dorósc(a)i "he surpasses" vs. ní-derscaigi "he does not surpass". This system is significantly more complex than the allomorphy present either in the preceding Primitive Irish period or the following Middle Irish period.
Contemporary Old Irish scholarship is still greatly influenced by the works of a small number of scholars active in the late 19th and early 20th centuries, among them Rudolf Thurneysen (1857–1940) and Osborn Bergin (1873–1950).
Old Irish was the only member of the Goidelic/Gaelic branch of the Celtic languages which is in turn a sub-family of the wider Indo-European language family that also includes Slavonic, Italic/Romance, Indo-Aryan, Germanic sub-families and a few less well known ones. Old Irish is the ancestor of all the modern Goidelic Langues; they are Modern Irish, Scottish Gaelic and Manx.
A still older form of Irish is known as Primitive Irish. Fragments of Primitive Irish, mainly personal names, are known from inscriptions on stone written in the Ogham alphabet. These inscriptions date from about the 4th to the 6th centuries. Primitive Irish appears to be very close to Common Celtic, the ancestor of all Celtic languages and has a lot of the characteristics of other archaic Indo-European languages.
Relatively little survives in the way of strictly contemporary sources. These are mainly represented by shorter or longer glosses on the margins or between the lines of religious Latin manuscripts, most of them preserved in monasteries in Germany, Italy, Switzerland, France and Austria, having been taken there by early Irish missionaries. Whereas in Ireland, many of the older manuscripts appear to have been worn out through extended and heavy use, their counterparts on the Continent were much less prone to the same risk, because once they ceased to be understood, they were rarely consulted.
The earliest Old Irish passages may be the transcripts found in the Book of Armagh and the Cambrai Homily, both of which are thought to belong to the early 8th century. Important Continental collections of glosses from the 8th and 9th century include the Würzburg Glosses (mainly) on the Pauline Epistles, the Milan Glosses on a commentary to the Psalms and the St Gall Glosses on Priscian's Grammar. Further examples are found at Karlsruhe (Germany), Paris (France), Milan, Florence and Turin (Italy). A late 9th-century manuscript from the abbey at Reichenau, now in St. Paul in Carinthia (Austria), contains a spell and four Old Irish poems. The Liber Hymnorum and the Stowe Missal date from about 900 to 1050.
In addition to contemporary witnesses, the vast majority of Old Irish texts are attested in manuscripts of a variety of later dates. Manuscripts of the later Middle Irish period, for instance, such as the Lebor na hUidre and the Book of Leinster, contain texts which are thought to derive from written exemplars in Old Irish now lost and retain enough of their original form to merit classification as Old Irish. The preservation of certain linguistic forms which were current in the Old Irish period may provide reason to assume that an Old Irish original directly or indirectly underlies the transmitted text or texts.
The consonant inventory of Old Irish is shown in the chart below. The complexity of Old Irish phonology is due to a four-way split of phonemes inherited from Primitive Irish, with both a fortis–lenis and a "broad–slender" (velarized vs. palatalized) distinction arising due to historical changes. The sounds /f v θ ð x ɣ h ṽ n l r/ are the broad lenis equivalents of broad fortis /p b t d k ɡ s m N L R/; likewise for the slender (palatalized) equivalents. (However, most /f fʲ/ sounds actually derive historically from /w/.)
|Plosive||broad||p b||t d||k ɡ|
|slender||pʲ bʲ||tʲ dʲ||kʲ ɡʲ|
|Fricative||broad||f v||θ ð||s||x ɣ||h|
|slender||fʲ vʲ||θʲ ðʲ||sʲ||xʲ ɣʲ||hʲ|
Some details of Old Irish phonetics are not known. /sʲ/ may have been pronounced [ɕ] or [ʃ], as in Modern Irish. /hʲ/ may have been the same sound as /h/ and/or /xʲ/. The precise articulation of the fortis sonorants /N/, /Nʲ/, /L/, /Lʲ/, /R/, /Rʲ/ is unknown, but they were probably longer, tenser, and generally more strongly articulated than their lenis counterparts /n/, /nʲ/, /l/, /lʲ/, /r/, /rʲ/, as in the Modern Irish dialects (e.g. Connacht Irish) that still possess a four-way distinction in the coronal nasals and laterals. /Nʲ/ and /Lʲ/ may have been pronounced [ɲ] and [ʎ] respectively. The difference between /R(ʲ)/ and /r(ʲ)/ may have been that the former were trills while the latter were flaps.
The inventory of Old Irish monophthongs is:
The distribution of short vowels in unstressed syllables is a little complicated. All short vowels may appear in unstressed final open syllables (an open syllable is one with no coda consonant), after both broad and slender consonants. The front vowels /e/ and /i/ are often spelled ae and ai after broad consonants, which might indicate a retracted pronunciation here, perhaps something like [ɘ] and [ɨ]. All ten possibilities are shown in the following examples:
|marbae||/ˈmarve/ or /ˈmarvɘ/||kill||2
|marbai||/ˈmarvi/ or /ˈmarvɨ/||kill||2
In unstressed closed syllables (that is, those with a syllable coda), the quality of a short vowel is almost entirely predictable by whether the surrounding consonants are broad or slender. Between two broad consonants, the vowel is /a/, as in dígal /ˈdʲiːɣal/ "vengeance" (nom.). Between a slender and a broad consonant the vowel is /e/, as in dliged /ˈdʲlʲiɣʲeð/ "law" (nom./acc.). Before a slender consonant the vowel is /i/, as in dígail /ˈdʲiːɣilʲ/ "vengeance" (acc./dat.), and dligid /ˈdʲlʲiɣʲiðʲ/ "law" (gen.). The chief exceptions to this pattern are that /u/ frequently appears when the following syllable contained an *ū in Proto-Celtic (for example, dligud /ˈdʲlʲiɣuð/ "law" (dat.) < PC *dligedū), and that /o/ or /u/ frequently appears after a broad labial (for example, lebor /ˈLʲevor/ "book"; domun /ˈdoṽun/ "world").
The inventory of Old Irish diphthongs is shown in this chart:
|Long (bimoraic)||Short (monomoraic)|
Old Irish was affected by a series of phonological changes that radically altered its appearance compared with Proto-Celtic and older Celtic languages (e.g. Gaulish, which still had the appearance of typical early Indo-European languages such as Latin or Ancient Greek). These changes were such that Irish was not recognized as Indo-European at all during much of the 19th century. The changes must have happened quite rapidly, perhaps in a one or two-century period c. 500 – 600 AD, since almost none of the changes are visible in Primitive Irish (4th to 6th centuries AD) and all of them are already complete in archaic Old Irish (8th century AD). A capsule summary of the most important changes is (in approximate order):
- Palatalization of all consonants before front vowels.
- Extensive umlaut ("affection") of short vowels, which were raised or lowered to agree with the height of following Proto-Celtic vowels. Similarly, rounding of *a to /o/ or /u/ often occurred adjacent to labial consonants.
- Lenition of all single consonants between vowels. This applied across word boundaries in the case of syntactically-connected words.
- Syllable-final *n (from PIE *m, *n) assimilated to the following phoneme, include across word boundaries in the case of syntactically-connected words.
- Voiceless stops became voiced: *mp *nt *nk > /b d g/.
- Voiced stops became prenasalized /mb nd ng/. (These were reduced to simple nasals after the Old Irish period.)
- Before a vowel, /n-/ was attached to the beginning of the syllable.
- Loss of part or all of final syllables.
- Loss of most interior vowels (syncope).
These led to the following effects:
- Both the palatalized ("slender") and lenited variants of consonants were phonemicized, multiplying the consonant inventory by 4 (broad, broad lenited, slender, slender lenited). *Variations between broad and slender became an important part of the grammar. e.g.:
- in masc. o-stems: macc "son" (nom. acc.) vs. maicc (gen.), cúl "back" (nom. acc.) vs. cúil (gen.), cf. Latin -us (nom.), -um (acc.) vs. -ī (gen.);
- in fem. ā-stems: túath "tribe, people" (nom.) vs. túaith (acc. dat.), mucc "pig" (nom.) vs. muicc (acc. dat.);
- in r-stems: athar "father" (gen.) vs. ath(a)ir (nom. acc. dat.).
- Lenition and nasal assimilation across word boundaries in syntactically-connected words produced extensive sandhi effects (Irish initial mutations). These variations became an important part of the grammar.
- Both umlaut (vowel affection) and especially syncope radically increased the amount of allomorphy found across declensions and conjugations. The most dramatic deviances are due to syncope: cf. as·berat "they say" vs. ní-epret "they do not say" or do·rósc(a)i "he surpasses" vs. ní-derscaigi "he does not surpass" (where the stressed syllable is boldfaced).
Examples of changes 
The following are some examples of changes between Primitive Irish and Old Irish.
|Primitive Irish||Old Irish||Meaning|
|Lugudeccas||Luigdech||genitive of Lug(u)id (name)|
|Anavlamattias||Anfolmithe||genitive of Anblamath (name)|
|Coillabotas||Coílbad||genitive of name|
Initial mutations 
All words within a syntactic phrase were treated phonologically as a single unit for the purpose of lenition and nasal assimilation, producing extensive sandhi effects. Synchronically these were unpredictable, but diachronically they reflected the state of the original final syllable in Primitive Irish:
- Lenition: If the word originally ended in a vowel, the first consonant of the following word was lenited.
- Gemination: If the syllable originally ended in *-s, or in *-t or *-k after a vowel, the first consonant was geminated, while /h-/ appeared before a vowel-initial word (regularly from *-s, analogically in the other cases). By Old Irish times, this gemination appeared only after vowel-final words.
- Nasalization (eclipsis): If the syllable originally ended in a nasal, the nasal is attached to the beginning of the following word, with various further changes (see above).
These mutations became an important part of the grammar, and remain with little change in modern Irish (see Irish initial mutations).
Mutations were only partly noted in Old Irish spelling:
- Lenition is only clearly indicated in the case of initial voiceless stops (written ph th ch) and (in later Old Irish) initial /f- s-/ (written ḟ ṡ).
- Nasalization is only clearly indicated in the case of initial voiced stops and vowels, where n- is prefixed (m- before b).
- Gemination is only partly indicated when geminated consonants were produced (geminated consonants were in the process of reducing to single consonants in Old Irish times). It was not indicated at all when /h-/ resulted.
The fact that mutations applied more extensively than indicated in spelling is largely inferred from later written and modern spoken evidence.
|Old Irish||Prim Irish ending||Old Irish||Prim Irish ending|
|Nom||fer becc /fʲer bʲegg/, fer cumachtach /k-/||*-os||fir bicc /fʲirʲ vʲigʲgʲ/, fir chumach(a)ig /x-/||*-ī < PIE *-oi|
|Voc||fir bicc /fʲirʲ vʲigʲgʲ/, fir chumach(a)ig /x-/||*-e||firu biccu /fʲiru bʲiggu/, firu cumachtchu /k-/||*-ūs < PIE *-ōs|
|Acc||fer m-becc /fʲer mbʲegg/, fer cumachtach /g-/||*-on < PIE *-om||firu biccu /fʲiru bʲiggu/, firu cumachtchu /k-/||*-ūs < PIE *-ōns|
|Gen||fir bicc /fʲirʲ vʲigʲgʲ/, fir chumach(a)ig /x-/||*-ī||fer m-becc /fʲer mbʲegg/, fer cumachtach /g-/||*-on < PIE *-ōm|
|Dat||fiur biucc /fʲĭŭr vʲĭŭgg/, fiur chumachtach /x-/||*-ū < *-ūi < PIE *-ōi||fer(a)ib becc(a)ib /fʲerǝvʲ bʲeggǝvʲ/, fer(a)ib cumachtch(a)ib /k-/||-obis < PIE *-obhis|
In the case of a "his, her, its, their", only the initial mutation of the following word distinguishes the various meanings.
|Effect||ech "horse"||bo "cow"||teg "house"||PIE form||Sanskrit form|
|Masc/Neut Sing||Lenition||a ech /a ex/||a bo /a vo/||a theg /a θʲeɣ/||*esyo||asya|
|Fem Sing||Gemination||a ech /a hex/||a bo /a bbo/||a teg /a ttʲeɣ/||*esyās < *esyeh₂s||asyās|
|Plur||Nasalization||a n-ech /a nex/||a m-bo /a mbo/||a teg /a dʲeɣ/||Masc/Neut *eysōm? *eysoHom? *eysom? (*es-?)
Fem *eys-? (*es-?) *ih₂s-?
|ēṡām (fem. āsām)|
These various changes, esp. syncope, produced quite complex allomorphy, because the addition of prefixes or various pre-verbal particles (proclitics) in Proto-Celtic changed the syllable containing the stress: According to the Celtic variant of Wackernagel's Law, the stress fell on the second syllable of the verbal complex, including any prefixes and clitics. By the Old Irish period, most of this allomorphy still remained, although it was rapidly eliminated beginning in the Middle Irish period.
Among the most striking changes are in prefixed verbs with or without pre-verbal particles. With a single prefix and without a proclitic, stress falls on the verbal root, which assumes the deuteronic ("second-stressed") form. With a prefix and also with a proclitic, stress falls on the prefix, and the verb assumes the prototonic ("first-stressed") form. Rather extreme allomorphic differences can result, e.g.:
|*ess-bero(n)t < PIE *-bheronti||as·berat /as-ˈbʲerəd/||they say||ní-epret /Nʲiː-ˈhebrʲəd/||they do not say|
|*cum-uss-ana||con·osna||he rests||ní-cumsana||he does not rest|
|*de-ro-uss-scochi||do·rósc(a)i||he surpasses||ní-derscaigi||he does not surpass|
|*de-lugi < PIE *-logheyeti||do·lug(a)i||he pardons||ní-dílg(a)i||he does not pardon|
|*de-ro-gn...||do·róna||he may do||ní-derna||he may not do|
The following table shows how these forms might have been derived.
|"they say"||"they do not say"||"he rests"||"he does not rest"||"he surpasses"||"he does not surpass"|
|Post-PIE||eks bheronti||nē eks bheronti||kom uks h₂eneh₂ti||nē kom uks h₂eneh₂ti||dē pro uks skokeyeti||nē dē pro uks skokeyeti|
|Proto-Celtic||eks ˈberonti||nī ˈeks-beronti||kom ˈuks-anāti||nī ˈkom-uks-anāti||dī ˈro-uks-skokīti||nī ˈdī-ro-uks-skokīti|
|Early Irish||ess ˈberont||nís ˈess-beront||kon ˈuss-anát||nís ˈkom-uss-anát||dí ˈro-uss-skokít||nís ˈdi-ro-uss-skokít|
|Palatalization||ess ˈbʲeront||nʲís ˈess-bʲeront||---||nʲís ˈkom-uss-anát||dʲí ˈro-uss-skokʲít||Nʲíh ˈdʲi-ro-uss-skokʲít|
|Lenition||---||Nʲíh ˈess-bʲeront||kon ˈuss-anáθ||Nʲíh ˈkow̃-uss-anáθ||dʲí ˈro-uss-skoxʲíθ||Nʲíh ˈdʲi-ro-uss-skoxʲíθ|
|Hiatus reduction||---||---||---||---||dʲí ˈróss-skoxʲíθ||Nʲíh ˈdʲi-róss-skoxʲíθ|
|Umlaut (vowel affection)||---||---||kon ˈoss-anáθ||Nʲíh ˈkuw̃-uss-anáθ||---||Nʲíh ˈdʲe-róss-skoxʲíθ|
|Loss of final consonant(s)||---||---||kon ˈoss-aná||Nʲíh ˈkuw̃-uss-aná||dʲí ˈróss-skoxʲí||Nʲíh ˈdʲe-róss-skoxʲí|
|Mora reduction in final vowel||---||---||kon ˈoss-ana||Nʲíh ˈkuw̃-uss-ana||dʲí ˈróss-skoxʲi||Nʲíh ˈdʲe-róss-skoxʲi|
|Nasal assimilation||ess ˈbʲerod||Nʲíh ˈess-bʲerod||---||---||---||---|
|Consonant assimilation||---||Nʲíh ˈeb-bʲerod||---||Nʲík ˈkuw̃-uss-ana||dʲí ˈrós-skoxʲi||Nʲíd ˈdʲe-rós-skoxʲi|
|Further consonant assimilation||---||Nʲíh-ˈebbrʲod||---||---||dʲí·ˈrósski||Nʲíd-ˈdʲerskoxʲi|
|Unstressed vowel reduction||ess·ˈbʲerǝd||Nʲíh-ˈebbrʲǝd||kon·ˈossna||Nʲík-ˈkuw̃ssǝna||di·ˈrósski||Nʲíd-ˈdʲerskǝxʲi|
|Geminate reduction (non-vocalic-adjacent)||as·ˈbʲerǝd||Nʲíh-ˈebrʲǝd||kon·ˈosna||Nʲík-ˈkuw̃sǝna||do·ˈróski||Nʲíd-ˈdʲerskǝxʲi|
|Fricative voicing between unstressed syllables||---||---||---||---||---||Nʲíd-ˈdʲerskǝɣʲi|
|Old Irish pronunciation||as·ˈbʲerǝd||Nʲíh-ˈebrʲǝd||kon·ˈosna||Nʲík-ˈkuw̃sǝna||do·ˈróski||Nʲíd-ˈdʲerskǝɣʲi|
|Old Irish spelling||as·berat||ní-epret||con·osna||ní-(c)cumsana||do·rósc(a)i||ní-(d)derscaigi|
The most extreme allomorphy of all came from the 3rd person singular of the s-subjunctive, because an athematic person marker -t was used, added directly onto the verbal stem (formed by adding -s directly onto the root). This led to a complex word-final cluster, which was deleted entirely. In the prototonic form (after two proclitics), the root was unstressed and thus the root vowel was also deleted, leaving only the first consonant. Examples:
|Present Indicative||Present Subjunctive|
|Positive (Deuterotonic)||Negative (Prototonic)||Positive (Deuterotonic)||Negative (Prototonic)|
|Primitive Irish||Old Irish||Primitive Irish||Old Irish||Primitive Irish||Old Irish||Primitive Irish||Old Irish|
|"he refuses"||*uss ˈbond-et(i)||as·boind||*nís ˈuss-bond-et(i)||ní op(a)ind /obǝnʲdʲ/||*uss 'bod-s-t||as·bó||*nís ˈuss-bod-s-t||ní op /ob/|
|"he remains over"||*di ˈwo-uss-ret-et(i)||do·fúarat||*nís ˈdi-wo-uss-ret-et(i)||ní díurat||*di ˈwo-uss-ret-s-t||do·fúair||*nís ˈdi-wo-uss-ret-s-t||ní diúair|
|"he repeats, amends"||*ad ˈess-reg-et(i)||ad·eirrig||*nís ˈ*ad-ess-reg-et(i)||(ní aithrig?? >) ní aithirrig||*ad ˈess-reg-s-t||ath·e(i)rr||*nís ˈad-ess-reg-s-t||ní aithir|
|"he can"||*con ˈink-et(i)||con·ic||*nís ˈcon-ink-et(i)||ní cum(a)ic > ní cum(u)ing, ní cumaing||*con ˈink-s-t||con·í||*nís ˈcon-ink-s-t, *nís ˈcon-ink-ā-t||ní cum, ní cumai|
|"it happens"||*ad ˈcon-ink-et(i)||(ad·cum(a)ic >) ad·cumaing||*nís ˈad-con-ink-et(i)||(ní ecm(a)ic >) ní ecmaing||*ad ˈcon-ink-ā-t||ad·cumai||*nís ˈad-con-ink-ā-t||ní ecm(a)i|
Additional details 
In more detail, syncope of final and intervocalic syllables involved the following steps (in approximate order):
- Loss of most final consonants, including *m, *n, *d, *t, *k, and all clusters involving *s (except *rs, *ls, where only the *s is lost).
- Loss of absolutely final short vowels (including those that became final as a result of loss of a final consonant).
- Shortening of long vowels in unstressed syllables.
- Collapsing of vowels in hiatus (producing new unstressed long vowels).
- Syncope (deletion) of vowels in every other interior unstressed syllable following the stress. That is, if there are two remaining syllables after the stress, the first one loses its vowel; if there are four remaining syllables after the stress, the first and third lose their vowel.
- Resolution of impossible clusters resulting from syncope and final-vowel deletion:
- Adjacent homorganic obstruents where either sound was a fricative became a geminate stop, voiceless if either sound was voiceless (e.g. *ðð *dð *ðd > /dd/; *θð *ðθ *θd *tθ etc. > /tt/).
- Otherwise, adjacent obstruents assumed the voicing of the second consonant (e.g. *dt > /tt/; *kd > /gd/; *ɣt > /xt/).
- *l *r *n not adjacent to a vowel became syllabic and then had a vowel inserted before them (e.g. domun "world" < *domn < *domnos < *dumnos; immormus "sin" < *imm-ro-mess). However, in the case of *n this occurred only when the nasal had not previously been joined to a following voiced stop as a result of nasal assimilation; c.f. frecnd(a)irc "present" (disyllabic).
- Remaining impossible clusters were generally simplified by deletion of consonants non-adjacent to vowels (e.g. between other consonants). Note, however, that Old Irish tolerated geminates adjacent to other consonants as well other other quite complex clusters, including e.g. , ainm /aNʲm/ "name" (one syllable), fedb /fʲeðβ/ "widow", do-aidbdetar /do-ˈaðʲβʲðʲǝdǝr/ "they are shown".
The following is a summary of some of the lower-level phonological changes yielding (written) Old Irish forms from Proto-Celtic (not in order).
- *φ is lost with compensatory lengthening of a preceding vowel.
- *j is lost in all positions after palatalizing a preceding consonant, except that before a dropped final vowel it survives as e.
- *kʷ *gʷ lose their labialisation (yielding c g).
- Initial *w becomes f. Intervocalic *w is lost with vowel colouring. *w is lost after *t *s and sometimes *d (always when initial), and becomes b after a voiced coronal.
- *t *d drop out before *b *s which are retained as p s. *d is lost in *ndw > nb.
- *ld and *ll can both be reflected as either ld or ll, and similarly for *nd and *nn. *ln yields ll.
- Medial *s assimilates to a following sonorant.
- Medial *st yields s.
- *zg yields dg.
- *rs yields rr.
- *x is lost before *s (and survives before *t, written ch).
- Medial *tr *br *dr yield tha(i)r ba(i)r da(i)r respectively.
- The first element of medial clusters *nt *nk *φm, or any medial cluster formed of *φ *t *k *b *d *g plus *l *n *r excepting *tr *br *dr, is lost with compensatory lengthening of any preceding vowel. Compensatorily lengthened vowels develop as original long vowels (below). Such clusters after *s still lose their first member but cause no lengthening.
- Consonants before *e *i *j, excepting the *k in *nk, become slender. If the consonant is not initial this is indicated by a preceding i in spelling (unless the preceding vowel is already i or í).
- ī, and *ē from compensatory lengthening, both yield either é or í, and similarly for *ū and *ō. The corresponding short vowels also can exchange before a consonant plus optional *j and a back vowel.
- *e *i before a consonant plus *(j)u can additionally yield iu. *ō *ū before a velar plus *(j)u can additionally yield úa.
- Nonfinally, the outcomes of *ei are é ía; of *ai and *oi are áe ái óe ói; of *eu and *ou is úa; of *au are áu ó.
- *iwo gives eó or ía; *ewo gives eó; *eiwi gives é; *eiwo gives ía; *awi gives á or ó; *awo gives ó; *owi gives úa.
- *e *i can yield respectively eu iu after *wl.
As with most medieval languages, the orthography of Old Irish is not fixed, so the following statements are to be taken as generalizations only. Individual manuscripts may vary greatly from these guidelines.
- a, b, c, d, e, f, g, h, i, l, m, n, o, p, r, s, t, u
- The acute accent indicates a long vowel. The following are long vowels: á, é, í, ó, ú.
- The superdot indicates the lenition of f and s: ḟ is silent, ṡ is pronounced /h/
- The superdot is also sometimes used on m and n with no change in pronunciation, when these letters are used to mark the nasalization mutation: ṁ, ṅ.
A number of digraphs are also used:
- The letter i is placed after a vowel letter to indicate that the following consonant was slender: ai, ei, oi, ui; ái, éi, ói, úi
- The letter h is placed after c, t, p to indicate a fricative: ch, th, ph
- The diphthongs are also indicated by digraphs: áe/aí, ía, uí, áu, óe/oí, úa, éu, óu, iu, au, eu
In word-initial position, when no initial consonant mutation has applied, the consonant letters have the following values; they are broad before back vowels (a, o, u) and slender before front vowels (e, i):
|Consonant||When no initial consonant mutation has applied in word-initial position before a, o, u||When no initial consonant mutation has applied in word-initial position before e, i|
|h||See discussion below|
Although Old Irish has both a sound /h/ and a letter h, there is no consistent relationship between the two. Vowel-initial words are sometimes written with an unpronounced h, especially if they are very short (the preposition i "in" was sometimes written hi) or if they need to be emphasized (the name of Ireland, Ériu, was sometimes written Hériu). On the other hand, words that begin with the sound /h/ are usually written without it, for example a ór /a hoːr/ "her gold". If the sound and the spelling cooccur, it is by coincidence, as ní hed /Nʲiː heð/ "it is not".
After a vowel or l, n, or r the letters c, p, t can stand for either voiced or voiceless stops; they can also be written double with either value:
|mac or macc||/mak/||son|
|bec or becc||/bʲeɡ/||small|
|op or opp||/ob/||refuse|
|brat or bratt||/brat/||mantle|
|brot or brott||/brod/||goad|
After a vowel the letters b, d, g stand for the fricatives /v, ð, ɣ/ or their slender equivalents:
After m, b is a stop, but after d, l and r it is a fricative:
|odb||/oðv/||knot (in a tree)|
After n and r, d is a stop
After n, l, and r, g is usually a stop, but it is a fricative in a few words:
|delg or delc||/dʲelɡ/||thorn|
|argat or arggat||/arɡǝd/||silver|
|bairgen||/barʲɣʲǝn/||loaf of bread|
After vowels m is usually a fricative, but sometimes a (nasal) stop, in which case it is also often written double:
|lom or lomm||/Lom/||bare|
The digraphs ch, ph, th do not occur in word-initial position except under lenition, but wherever they occur they are pronounced /x/, /f/, /θ/.
The letters l, n, and r are written double when they indicate the tense sonorants, single when they indicate the lax sonorants. (But the tense sonorants are usually written single in word-initial position.)
Written vowels a, ai, e, i in post-stressed syllables (except absolutely word-finally) all seem to represent phonemic /ǝ/. The particular vowel that appears is determined by the quality (broad vs. slender) of the surrounding consonants, and has no relation to the etymological vowel quality:
- a appears between two broad consonants;
- i appears between two slender consonants;
- ai appears between broad and slender;
- e appears between slender and broad.
The written vowel may approximate the actual phonetic quality of phonemic /ǝ/.
Old Irish had three genders, namely, masculine, feminine and neuter; three numbers, namely, singular, dual and plural, with the dual being attested only to a limited degree with somewhat distinct forms, though it is almost always preceded by the cardinal dá, meaning "two" (and as such has been retained in the modern Gaelic languages); and five cases (nominative, vocative, accusative, genitive and dative). Thurneysen had fourteen classes of noun, defined by the morphological marking on the stem, with seven vocalic stems and seven consonantal stems (including one class of irregular and indeclinable nouns). The full range of case is only evident in the noun phrase, where the article causes noun initial mutation, and where the initials of following adjectives are mutated according to the underlying case ending, thus in fer becc "the small man", nominative, differs from the accusative in fer /βer/ m-becc, though at times such mutations were not written. In the following H shows lenition of the following adjective, N shows nasalization (eclipsis) of the following adjective, and h shows prefixing of h to following vowel initial adjectives. (These mutations are related to the form of the case ending in Common Celtic. Endings with a final vowel triggered lenition; those with a final nasal consonant triggered eclipsis; and those with a final /s/ triggered prefixing of h. For the most part, the endings that can be reconstructed by these mutations agree with the corresponding forms in Sanskrit, Ancient Greek and/or Latin.)
|Nominative/Vocative||túath H||túaith H||túatha|
|Genitive||túaithe (h)||túath N|
|Vocative||fir H||fer||firu h|
|Accusative||fer N||firu h|
|Genitive||fir H||fer N|
Verbs stand initially in the sentence (preceded only by some particles, forming a "verbal complex", and very few adverbs). The verb can be either suffixed for tense, person, mood and aspect (often portmanteau suffixes), or these can be shown by vowel changes in the stem (e.g. as·beir present "says", as·rubart past "said", as·béra future "will say"). Before this core "verb phrase" are placed various other preverbal clitic particles, e.g. negative ni-/ní-, perfective ro- or one or more preverbal particles that modify the meaning of the verb in unpredictable ways (compare ā-, ex-, in-, dē-, etc. in Latin verbs). Personal pronouns as direct objects are infixed between the preverb and the verbal stem. In an overall sense, the verb structure is agglutinative. A single verb can stand as an entire sentence in Old Irish, in which case emphatic particles such as -sa and -se are affixed to the end of the verb.
Verbs are conjugated in present, imperfect, past, future and preterite tenses; indicative, subjunctive, conditional and imperative moods; and active and passive voices. The only verbal form lacking in Old Irish is the infinitive, this covered, as in the modern Gaelic languages, by the verbal noun. Old Irish inherits a large amount of Indo-European verbal morphology, including:
- extensive ablaut variations, made significantly more complicated by vowel affection and syncope;
- primary and secondary endings;
- thematic and athematic endings;
- deponent verbs;
- multiple ways of forming each of the various tenses and moods, and no general cross-tense/mood conjugational classes. (That is, in general a series of principal parts must be memorized for each verb, much as in Latin or Ancient Greek.)
Most verbs have, in addition to the tenses, voices, and moods named above, two sets of forms: a conjunct form, and an absolute form. The absolute form occurs when the verb occurs absolutely sentence-initial with no preverbs, while the conjunct form occurs when the verb is preceded by one or more preverbs. Absolute and conjunct forms are distinguished primarily by the endings. In addition, the conjunct form comes in two variants, deuterotonic ("second-stressed", when exactly one preverb precedes and the stress is on the first syllable of the verbal stem) and prototonic ("first-stressed", when more than one preverb precedes and the stress is on the second preverb). These variants are marked by (sometimes radical) changes in the verbal stem and non-initial preverbal particle(s), which merge with the stem. This is due to the Celtic version of Wackernagel's law, where stress falls in second position whenever there is one or more preverbs.
The difference between absolute and conjunct endings is thought to reflect an additional particle *-es added to the absolute verbal form. Final -i in the conjunct forms was apparently lost early on (cf. a similar change in Latin).
See below for an example of absolute vs. conjunct endings, and deuteronic vs. prototonic stems.
Present tense 
The following is an example of a strong present-tense verb (class B I), showing the absolute, conjunct deuterotonic and conjunct prototonic forms.
|Old Irish||PCelt||Old Irish||PCelt||Old Irish|
|3rd Sing||berid, -ith||*bereti-s||do⋅beir||as⋅beir||*-beret(i)||ní-tab(a)ir||ní-ep(a)ir|
|2nd Pl||*beirthe||*beretes-es||do⋅berid, -ith||as⋅berid, -ith||*-beretes||ní-taibrid, -ith||ní-eprid, -ith|
The following present-tense formations existed:
- A I: a-presents (e.g. mór(a)id "he magnifies", conjunct ·móra), with a suffix *-ā- < PIE *-eh₂ (cf. Latin -āre)
- A II: i-presents (e.g. lé(i)cid "he leaves", conjunct ·lé(i)ci), with a suffix *-ī- < PIE causative *-éye- with o-grade, PIE denominative *-eyé-, PIE stative *-eh₁ (cf. Latin -ēre, -īre)
- A III: hiatus verbs (e.g. raïd/ráïd "he rows", conjunct ·rá; gniïd/gníïd "he does"), with a root that synchronically ends with a vowel
- B I: Verbs with alternating root-final broad/slender quality (e.g. berid "he carries", agid "he drives", canid "he sings") < PIE simple thematic verbs
- B II: Verbs originally with consistent root-final slender quality (e.g. a(i)rid "he plows", ga(i)bid "he takes", gu(i)did "he prays") < PIE thematic verbs in *-y-
- B III: Verbs with n-infix (e.g. bongid "he breaks", with reduplicated preterite bobag-) < PIE n-infix verbs
- B IV: Verbs with broad n-suffix (e.g. cren(a)id "he buys", cf. Sanskrit krīṇā́ti, 3rd sing. subjunctive ·cria) < PIE -neh₂- verbs
- B V: Verbs originally with alternating broad/slender n-suffix (e.g. ara·chrin "he decays", pl. ara·chrinat) < PIE -neu-/-nu- verbs
Other forms 
The subjunctive comes in two variants:
- a-subjunctive (cf. Latin subjunctives in -ā-)
- s-subjunctive, apparently < PIE s-aorist subjunctive
In the s-subjunctive, the s is attached directly to the root. The endings are partly athematic, especially the 3rd singular, with original suffix *-s-t that leads to truncation of the root: cf. as·boind "he refuses", prototonic ·op(a)ind; subj. as·bó, prototonic ·op; 2 sg. subj. as·bóis, prototonic ·obbais with thematic *-s-es.
The imperfect is built off the same stem as the present, but with different endings. The same endings are used in the past subjunctive, attached to the present subjunctive stem.
The future comes in four variants:
- f-future, made to weak verbs;
- reduplicated a-future;
- é-future (é replaces verb-stem vowel);
- reduplicated s-future (cf. Sanskrit desiderative jighāṃsati "he wants to kill" < PIE *gʷhi-gʷhn̥-h₁s-eti, root *gʷhen-).
The preterite comes in four variants:
- reduplicated suffixless preterite;
- non-reduplicated suffixless preterite.
Old Irish follows the typical VSO (verb-subject-object) structure shared by most Insular Celtic languages (even though other orders are possible, especially under Bergin's Law). Verbs are all fully conjugated, and have most of the forms typical of Indo-European languages (see above). Personal pronouns, when used as direct objects, are prefixed to the verb with which they are associated (after other prefixes, and therefore are often referred to as infixes). Prepositions have the same status as the Latin prepositions, including the property of being verb prefixes.
See also 
- Early Irish literature
- Dictionary of the Irish Language
- Auraicept na n-Éces
- Goidelic substrate hypothesis
- Koch, John Thomas (2006). Celtic culture: a historical encyclopedia. ABC-CLIO. p. 831. "The Old Irish of the period c. 600–c. 900 AD is as yet virtually devoid of dialect differences, and may be treated as the common ancestor of the Irish, Scottish Gaelic, and Manx of the Middle Ages and modern period; Old Irish is thus sometimes called 'Old Gaelic' to avoid confusion."
- Ó Baoill, Colm (1997). "13: The Scots-Gaelic Interface". The Edinburgh History of the Scots Language. Edinburgh University Press. p. 551. "The oldest form of the standard that we have is the language of the period c. AD 600-900, usually called 'Old Irish' - but this use of the word 'Irish' is a misapplication (popular among English-speakers in both Ireland and Scotland), for that period of the language would be more accurately called 'Old Gaelic'."
- Thurneysen, A Grammar of Old Irish, p. 4.
- Thurneysen, p. 18.
- Thurneysen, p. 137.
- Thurneysen, p. 181.
- Thurneysen, p. 58.
- Thurneysen, p. 98.
- Thurneysen, pp. 192-93.
- Thurneysen, p. 42.
- Sihler, Andrew (1995). New Comparative Grammar of Greek and Latin. Oxford University Press. P. 391.
- Per Sihler (1995), Ringe (2006). *eso according to Beekes (1995). See footnote for genitive plural.
- The PIE form of the genitive plural (both ending and stem) is somewhat unclear. The ending is traditionally reconstructed as *-ōm, but Don Ringe (From Proto-Indo-European to Proto-Germanic, 2006, p. 56) reconstructs *-oHom, while both Andrew Sihler (New Comparative Grammar of Greek and Latin', 1995, p. 391) and Robert Beekes (Comparative Indo-European Linguistics: An Introduction, 1995, pp. 213, 229) reconstruct *-om. Ringe and Sihler both reconstruct a masculine/neuter stem *eys-, while Beekes reconstructs *es-. Sihler and Beekes both reconstruct the feminine stem as identical to the masculine stem (*eys- for Sihler, *es- for Beekes), while Ringe reconstructs a separate feminine stem *ih₂s-. These differences are based on the divergent attested outcomes, with the various scholars differing as to which parts of which attested forms are inherited and which are due to analogy.
- Thurneysen, A Grammar of Old Irish, p. 68.
- Fortson, Benjamin W., IV. "Celtic". Indo European Language and Culture: An Introduction. P. 324.
- Thurneysen, A Grammar of Old Irish, pp. 70, 100.
- Pages x-xxxi of An etymological dictionary of the Gaelic language by Alexander Macbain, first published 1896.
- Thurneysen, Rudolf (1946). A Grammar of Old Irish. Dublin Institute for Advanced Studies. P. 363.
- Sihler, Andrew (1995). New Comparative Grammar of Greek and Latin. Oxford University Press. P. 465.
- Green, Antony (1995). Old Irish Verbs and Vocabulary. Somerville, Massachusetts: Cascadilla Press. ISBN 1-57473-003-7.
- Lehmann, R. P. M.; W. P. Lehmann (1975). An Introduction to Old Irish. New York: Modern Language Association of America. ISBN 0-87352-289-3.
- McCone, Kim (1987). The Early Irish Verb. Maynooth: An Sagart. ISBN 1-870684-00-1.
- McCone, Kim (2005). A First Old Irish Grammar and Reader. Maynooth: Department of Old and Middle Irish, National University of Ireland. ISBN 0-901519-36-7.
- O’Connell, Frederick William (1912). A Grammar of Old Irish. Belfast: Mayne, Boyd & Son.
- Quin, E. G. (1975). Old-Irish Workbook. Dublin: Royal Irish Academy. ISBN 0-901714-08-9.
- Stifter, David (2006). Sengoidelc: Old Irish for Beginners. Syracuse, New York: Syracuse University Press. ISBN 0-8156-3072-7.
- Strachan, John (1949). Old-Irish Paradigms and Selections from the Old-Irish Glosses. Revised by Osborn Bergin (Fourth ed.). Dublin: Royal Irish Academy. ISBN 0-901714-35-6.
- Thurneysen, Rudolf (1993) . A Grammar of Old Irish. Translated by D. A. Binchy and Osborn Bergin. Dublin: Dublin Institute for Advanced Studies. ISBN 1-85500-161-6.
- Tigges, Wim; Feargal Ó Béarra (2006). An Old Irish Primer. Nijmegen: Stichting Uitgeverij de Keltische Draak. ISBN 90-806863-5-2.
|For a list of words relating to Old Irish, see the Old Irish language category of words in Wiktionary, the free dictionary.|
- An Etymological Dictionary of the Gaelic Language MacBain, Alexander Gairm Publications, 1982
- Old Irish dictionary
- Old Irish Online from the University of Texas at Austin.
- eDIL (digital edition of the Dictionary of the Irish Language)