O'Rahilly's historical model
O'Rahilly's historical model is a theory of Irish prehistory put forward by Celtic scholar T. F. O'Rahilly in 1946. It was based on his study of the influences on the Irish language and a critical analysis of Irish mythology.
He distinguished four separate waves of Celtic invaders:
- The Cruithne or Priteni (c. 700 – 500 BC)
- The Builg or Érainn (c. 500 BC)
- The Laigin, the Domnainn and the Gálioin (c. 300 BC)
- The Goidels or Gael (c. 100 BC)
O'Rahilly's work was and remains influential but much of his linguistic work has since been refuted by noted authors such as Kenneth Jackson and John T. Koch and is not generally the accepted model.
Nevertheless, and independent of his linguistic arguments, O'Rahilly's categorizations of most Irish kin groups generally remain accepted, although with important exceptions, e.g. those he believed were the true Gaels cannot actually be demonstrated to be. In any case it is this historical aspect of his work which is most frequently cited in current scholarship. In recent decades, the developing understanding of Irish prehistory has mostly reduced the influence ascribed to "invasions" or mass movements in population.
The Pretanic colonisation 
According to O'Rahilly's model, between 700 and 500 BC, iron-wielding, Celtic-speaking people first settled in Britain and Ireland from the continent. They spoke a P-Celtic tongue, and called themselves Priteni or Pritani. The impact they had upon the native inhabitants can be inferred from the fact that Greek geographers were referring to these islands as the “Pretanic Islands” (αι Πρετανικαι νησοι) by at least 300 BC. It is also possible that the very name “Britain” is derived from Priteni. However, there is no hard evidence for a Pretanic invasion as such. It is much more likely that their settlement of these islands was a gradual one, spread over several centuries.
In Britain these Priteni were absorbed by later invaders and lost their cultural identity, except in the far north where they were known to the Romans as Picti, or “painted people,” on account of their practice of decorating their bodies with tattoos (a practice which by then had died out among other Celtic nations). In Ireland, too, the Priteni were largely absorbed by later settlers; but a few pockets of them managed to retain a measure of cultural, if not political, independence well into the Christian era. By then they were identified as Cruithne or Cruthin, a Q-Celtic adaptation of the P-Celtic Priteni. Both words are derived from a root meaning “to shape” or “create.” Celtic tribes generally gave themselves names which were the pluralised forms of names they gave to their deities (in this case “the Creator”). Among the Cruthnian tribes that survived into the Christian era the most prominent were the Dál nAraidi in Ulster, and the Loíges and Fothairt in Leinster. The name of the second of these tribes, modernized as Laois, has been revived and given to one of the counties of Leinster (formerly known as Queen's County).
The Bolgic or Ernean invasion 
Around 500 BC the Cruthin lost their dominant position in Irish society when the country was invaded by a second wave of P-Celtic speakers. These were the Builg or Érainn. The former name (originally Bolgi) identifies them as Belgae,[verification needed] a Celtic people mentioned by Julius Caesar in Commentarii de Bello Gallico. Their other name (originally Iverni) is probably the origin of several of the early Classical names for Ireland: the Greek Ιερνη; Ιουερνια and possibly also the Latin Hibernia. In Irish mythology the name Fir Bolg obviously refers to the same people. It appears that groups of these Belgae colonised Britain and Ireland in the late 6th or early 5th century. In both islands they absorbed and subjugated most of the previous inhabitants. According to their own traditions the Érainn arrived in Ireland from Britain, and there is no good reason to dispute this.
Among the more prominent Ernean tribes were the following:
- The Uluti (Middle Irish: Ulaid), after whom Ulster is named. For centuries the Uluti were the dominant tribe in the north of the country. They founded Emain Macha (Navan Fort), the traditional capital of Ulster, near Armagh. In later historical times their descendants were known as the Dál Fiatach and were confined to the eastern half of County Down.
- The Darini and Robogdii (or Reidodioi), two tribes whose territory was in Antrim and north Down. They were probably the ancestors of the historical Dál Riata or Dál Riada, who colonised Scotland in the 5th century of the common era and from whom the kings of Scotland were descended.
- The Iverni, the dominant Ernean tribe in Munster from whom the Érainn as a whole took their name. The Corcu Loígde of historical times were their descendants.
- The Ebdani, a tribe of the east coast whose name appears as Eblani in Ptolemy's description of Ireland in his Geographia. Their settlement, which Ptolemy calls Eblana, has often been mistakenly identified with the city of Dublin on account of the similarity of the two names.
The Laginian invasion 
About two centuries after the Bolgic invasion Ireland was subjected to another invasion of P-Celtic-speaking people. Three names can be distinguished for them, but whether they were one tribe with three different names or three closely allied but separate tribes we cannot say. These names, as given in later written records, are Laigin, Domnainn and Gálioin. According to their own traditions, they came to Ireland from Armorica (Brittany). They landed in the southeast of the country and took the southeastern quarter from the Érainn. The modern name of this province, Leinster (Irish: Laighin), preserves the memory of this Laginian conquest, although in ancient times it was much smaller than the modern province. Before the Goidelic invasion, the River Liffey marked the boundary between Ulster and Leinster. The Domnainn were clearly a branch of the Dumnonii, a Celtic people identified by Classical authors as inhabiting Dumnonia (the English counties of Cornwall and Devon, to which they gave their name). Another branch of the Dumnonii settled in Scotland, where they founded Dumbarton and established the kingdom later known as Strathclyde. Presumably these settlements occurred at around the same time as the Laginian invasion of Ireland. It is even possible that the Dumnonii of Scotland were originally Irish Domnainn.
The Laginian invasion made little impact in Ulster or Munster, where Ernean tribes continued to be the dominant force. But the same cannot be said for Connacht, the westernmost of the four provinces. Sometime in the 3rd century (all these dates, it should be pointed out, are highly conjectural) they crossed the River Shannon and subjugated the Ernean tribes of Connacht. The decisive battle was fought in County Sligo, in a place called Mag Tuired (Moytura). There a Laginian king (possibly known as Cairbre) overthrew the Érainn and drove them out of Connacht. According to Irish records the defeated Érainn sought refuge in many of the islands around Ireland. The fortresses of Dún Aengus and Dún Conor on the Aran Islands, and Dún Balor on Tory Island, are thought to have been built by them.
It was probably as a result of the Laginian conquests that the island of Ireland first came to be divided into four provinces. The Érainn continued to rule in Ulster and Munster, while the Lagin and their allies became the dominant force in Leinster and Connacht. Traditionally these four provinces met at the exact centre of the country, which was marked by the Hill of Uisneach (between Mullingar and Athlone in County Westmeath), a name which may mean “vertex” or “angular place.” The district immediately surrounding this hill was originally called Medion, which means “middle,” and is the origin of the county-name Meath. Julius Caesar informs us that the druids of Gaul regularly assembled at a hallowed spot in the centre of the country to celebrate their rituals (De Bello Gallico 6.13). Irish tradition records that a similar assembly, the Mórdáil Uisnig, periodically took place at the Hill of Uisneach on Beltane, the May-day festival.
The Goidelic invasion 
The fourth and final Celtic invasion of Ireland was the Goidelic or Gaelic invasion. Unlike the previous invaders, the Goidels spoke a Q-Celtic language, which was the forerunner of Modern Irish, Scots Gaelic and Manx. The P-Celtic dialects which were spoken in the country at the time of their arrival (and which they referred to as Iarnbélre, “language of the Érainn”) eventually became extinct. The Goidels originated in Aquitania in southwestern Gaul. Around 100 BC two groups of these Celts emigrated to Ireland:
The Connachta were named after Conn Cétchathach, or Conn of the Hundred Battles; a mythical ancestor who was later euhemerized and given a place in Irish history. The Connachta were led by a man known to later history as Tuathal Teachtmhar. They landed at the mouth of the Boyne and pushed inland to Tara, the seat of the local Ernean king, which they sacked. They soon carved out for themselves a new province between Ulster and Leinster, running from the mouth of the Liffey to the mouth of the Boyne and inland as far as the Shannon. In time, this fifth province came to be known as “Meath,” presumably for no other reason than that it contained the Hill of Uisnech.
The other group were known as the Eóganachta. Their leader is known to history as Mogh (or Mug) Nuadat. The Eoganachta landed at a place called Inber Scéne, usually identified with Kenmare River in the southwest of the country. Unlike the Connachta, they did not carve out a new province for themselves. Instead, they slowly but gradually rose in power, eventually becoming the dominant force in Munster. The names Eóganachta and Mug Nuadat, which were probably adopted after their arrival, suggest that initially there were friendly relations between them and the Érainn, as both names are derived from the titles of Ernean deities. Early in their history, the Connachta subjugated the Laginian tribes of Leinster and reduced them to a state of vassalage. The Laginian leaders were allowed to retain possession of their territory, but a heavy tribute was imposed upon them. Known as the Bórama (or Bórú), it continued to be exacted at irregular intervals until the 8th century of the common era. According to one tradition, it was Tuathal Techtmar himself who first imposed this tribute on the Lagin.
As their name suggests, the Connachta did not stop when they reached the Shannon. At an indeterminable point in history, some of them crossed the Shannon and conquered the western province, bestowing their name on it in the process. By the late 4th century of the common era Ireland was divided politically into five provinces or "overkingdoms":
- Ulster was still dominated by Ernean tribes
- Meath, or the Midland territory, included Tara and was ruled by Gaelic tribes
- Leinster was held by Gaelicized tribes subject to Meath
- Munster was ruled by Gaelic tribes
- Connacht was ruled by Gaelic tribes
As can be seen, most of the country was in the hands of the Gael. Only Ulster remained independent, but this was not to last.
- O'Rahilly, T. F. "Early Irish History and Mythology." Dublin: Dublin Institute for Advanced Studies, 1946. (reprinted 1964, 1971, 1984) ISBN 0-901282-29-4
- Language and History of Early Britain, Kenneth H. Jackson, Edinburgh: Edinburgh University Publications (1953)
- J T Koch Ériu, Alba, Letha: When Was a Language Ancestral to Gaelic First Spoken in Ireland?, Emania IX (1991)