Fáil Inis was a hound-whelp owned by Lugh Lámhfhada of the Tuatha Dé Danann in the Mythological Cycle of Irish literature. The hound was invincible in battle, caught every wild beast it encountered, and could magically change any running water it bathed in into wine. It was one of the prizes exacted from the children of Tuireann (Brían, Iuchar and Iucharba) as reparation for the slaying of Lugh's father Cian.
By the same name, or nearly so (Failinis or Ṡalinnis), this hound of Lugh also occurs in recensions of a certain medieval ballad from the Fenian cycle. The ballad relates how the wonder-dog is brought to Ireland by a threesome (also from "Iruaid"). A variant telling of this threesome also occurs in The Colloquy of the Elders, though the dog's name there is Fer Mac.
Before the sons of Tuireann obtained Fáil Inis, the pup belonged to the king of Ioruiadhe (or Iruaid, Irúaith, etc.) (≒ Legendary Scandinavian kingdom), and about this hound it was said that "all the wild beasts of the world […] would fall down out of their standing" (i.e., prostrate themselves) and that it was "more splendid than the sun in his fiery wheels." The hound's hide or pelt moreover possessed the ability to turn water into wine.
The threesome from Ioruaidh encountered by the Fianna also possessed the same dog, Failinis, formerly owned by Lugh. These three used Failinis as a boundless source of wine as well as guard dog. The hound slew one of the Fianna, and as reparation, it was exterminated and flayed for its hide/pelt.
Aided Chloinne Tuireann 
The hound's name Failinis appears in the full narrative version of The Fate of the Children of Tuireann (see Brian (mythology), Tuireann), which only survives in manuscripts from early 18th century and later. However, neither dog nor its name is by any means a concoction of late tradition. The Lebor Gabála Éirenn (Book of Invasions) gives a brief account of Lugh's demands, according to which he obtained "the whelp of the royal smith of Ioruath" (Irish: Cuilen rīg goband na Hiruaidhe) which was "a hound by night and a sheep by day" and whatever pool of water touched its hide (Irish: croccenn) turned to wine. The hound's name is not specifically given in the Book of Invasions.
12th-century ballad 
However, the name of Lugh's dog Failinis is indeed recorded in Medieval manuscripts. It occurs in a certain "ballad" (Irish: dúan) starting with the line "They came a band of three…" (Irish: "Dám Thrír Táncatair Ille). It has been characterized by Stern as an Ossianic ballad of the 12th century, i.e., a poem in the pretext of a work by Oisín in reminiscence of the Fianna's past. The ballad relates how a threesome from Iruaid brings along a magical dog (Irish: Ṡalinnis (Shalinnis) (LL version) / Failinis (Lismore version))) which turns any fresh water (spring water) it touches into mead or wine. The dog once belonged to Lugh of the mantles (Irish: Lugh na Lenn, a corruption of Lugh's matronymic "Lugh mac Ethlenn"). The hound is responsible for the death of one of the Fianna (Dubán mac Bresail) so that the threesome (Sela, Dorait, Domnán) forfeit the dog as compensation. The Fianna then kill the hound and flay its hide (presumably with its wine-making powers intact), and carry it off into foreign campaigns.
Colloquy of the Elders 
The Acallam na Senórach (The Colloquy of the Elders), which is narrated by Caílte, also describes a threesome from Iruaid (Ioruaidh, Irúaith) met by the Fianna, and though they have different names (Dub, Ág, Ilar) they must represent the same characters. Their wonder-dog, called here Fermac (or Fer Mac), is not specifically said to have been Lugh's in the past. But it manifests qualities that are similar to it. Here, Fermac was a parti-colored furred dog, which belched out wine as well as gold and silver. Though it was a huge dog by day, it shrunk to the size of a lapdog by night. Two watchmen (Ulster princes attached to the Fianna), in breach of the threesome's specific warning, spied on them at nighttime, but suffered the consequences. The lapdog swirled a druidic wind with its tale, and the watchmen were swept away, their swords driven into each other's bodies.
- Ellis 1987, p. 145.
- Ellis 1987, p. 112.
- Onomasticon Goedelicum, "hirotae"
- O'Curry, ed. tr., Fate of the Children of Tuireann, Atlantis IV p.162/3
- Lebor Gabhala, Sect. 319 and the 12th-century ballad (detailed below)
- Ballad "They came a band of Three" (see "Primary sources"); cf., 4 stanzas excerpted in O'Curry, Atlantis IV, 396-7 "The whelp of the King of Iruaidhe […] that had been with Lugh of the Mantles… Mead or wine would grow of it, / Should she bathe in spring water."
- Stokes tr., Irische texte, p.237. Supplementary translation is given here because it is wanting in Standish O'Grady, Silva Gadelica
- Dooley and Roe's translation, p. 156
- Macalister ed., ¶319 and poem LXVI. Macalister's translation of the prose part skips the word "hide" but in the poem LXVI, strophe 14. "whelp—…of the royal smith of Iruaith, wine would be every water,…which is put upon its skin.")
- Stern ed. tr., in Festschrift Wh. Stokes (see "Primary sources")
- Stokes ed. ZCP 3, p.432
- pointed out by Stern, op. cit.
- Stokes ed. tr., Dooley & Roe tr.(Primary sources)
- see Dooley & Roe Tales of the Elders of Ireland, p.153
- Ellis, Peter Berresford (1987), A Dictionary of Irish Mythology, Oxford: Oxford University Press, ISBN 0-19-282871-1
- Mackillop, James, Dictionary of Celtic Mytholgy (1998)
- Lebor gabála Érenn, R.A.S. Macalister ed., tr., Book 4, "Part VII: Invasion of the Tuatha De Danann" ¶319, Poem no. LXVI.
- The Fate of the Children of Tuireann ([A]oidhe Chloinne Tuireann), Eugene O'Curry ed., tr., in Atlantis IV, pp. 162-
- Ballad "They Came a Band of Three" ("Dám Thrír Táncatair Ille")
- (4 strophes only) O'Curry ed. tr. The Atlantis III, London 1862, 396-7 (The hound's name is wanting in excerpt. "Lismore fol. 194" is incorrect.)
- (LL 207b) Stern, L. Chr., ed. tr. (German) "Eine ossianische Ballade aus dem XII. Jahrhundert", Festschrift Whitley Stokes zum siebzigsten Geburtstage, 1900, pp. 7–12
- (Book of Lismore, fo. 153 b). Stokes ed.,Zeitschrift für Celtische Philologie, III, p. 432
- Acallamh na Senórach
- Stokes tr. "The Story of the Oakgrove of Conspiracy" (Text: lines 6083-6141), Acallamh na Seanórach, Irische Texte III, p. 237-
- Dooley & Roe tr., Tales of the Elders of Ireland, p. 153 (and endnote); p. 171ff