[Ir., man of the sea].
Fictional early king of Ireland invented by medieval genealogists. Fiachu's father, Angus Tuirmech, lies with his own daughter while drunk. To conceal the shame of his incest, Angus takes the infant Fiachu, still dressed in royal purple, and puts him to sea in a small boat off the Donegal coast. He is rescued by fishermen who raise him under the name Fer Mara [man of the sea]. Despite or perhaps because of his fabulous origin, Fiachu's name is cited in many genealogies; he was thought to be the father of Eterscél and thus the grandfather of Conaire Mór. The incestuous birth and infant voyage is an Irish instance of the international tale type 933, most often associated with the fabricated story of the birth of Pope Gregory. R. S. Loomis (1927) noted that the story has striking parallels with that of the conception of the Arthurian figure Mordred.