Fionn mac Cumhaill
It was originally the a hound-whelp of the smith or the king of Iruaith (Ioruath, Hiruaidhe, etc.). Later on, Lugh's Failinis (var. Fer Mac) belonged to a foreign threesome from Iruaith that came to Ireland, and encountered by the Fíanna led by Fionn mac Cumhaill in the Fenian cycle.