He is known primarily from the saga Forbhais Droma Dámhgháire, in which he is assisted by the famous Mug Ruith, who repels an invasion of his kingdom by Cormac mac Airt.
Like his father Éogan Mór, grandfather Ailill Aulom, and great-grandfather Mug Nuadat, Fiachu Muillethan appears to be mainly fictional.
The circumstances of his life are entirely legendary, seemingly invented so as to provide the Eóganachta with an ancestral contemporary of Cormac mac Airt.
While the career of Fiachu Muillethan may be entirely legendary, that of his supposed great-grandson Conall Corc, the true founder of the Eóganachta, may preserve an amount of historical fact.
The medieval genealogists are at pains to assert they were identical but base their argument only on the story of Fiachu's birth in the Cath Maige Mucrama, a well known saga of political fiction.