Wikipedia
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 Ioruaidhe (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.
The dog belonging presumably to same trio from Ioruaidh also makes appearance Colloquy of the Elders, but there, the dog is called Fermac (or Fer Mac).