Fiachu Muillethan or Fiachu Fer Da Liach (of the two sorrows), son of Éogan Mór, was a legendary king belonging to the Deirgtine

Deirgtine

The Deirgtine or Clanna Dergthened were the proto-historical ancestors of the historical Eóganachta dynasties of Munster. Their origins are unclear but they may have been of fairly recent Gaulish derivation...

, the proto-historical ancestors of the Eóganachta

Eóganachta

The Eóganachta or Eoghanachta were an Irish dynasty centred around Cashel which dominated southern Ireland from the 6/7th to the 10th centuries, and following that, in a restricted form, the Kingdom of Desmond, and its offshoot Carbery, well into the 16th century...

 dynasties of Munster

Munster

Munster is one of the Provinces of Ireland situated in the south of Ireland. In Ancient Ireland, it was one of the fifths ruled by a "king of over-kings" . Following the Norman invasion of Ireland, the ancient kingdoms were shired into a number of counties for administrative and judicial purposes...

. He is known primarily from the saga Forbhais Droma Dámhgháire, in which he is assisted by the famous Mug Ruith

Mug Ruith

Mug Ruith is a figure in Irish mythology, a powerful blind druid of Munster who lived on Valentia Island, County Kerry. He could grow to enormous size, and his breath caused storms and turned men to stone. He wore a hornless bull-hide and a bird mask, and flew in a machine called the roth rámach,...

, who repels an invasion of his kingdom by Cormac mac Airt

Cormac mac Airt

Cormac mac Airt , also known as Cormac ua Cuinn or Cormac Ulfada , was, according to medieval Irish legend and historical tradition, a High King of Ireland...

. The sons of Fiachu Muillethan were Ailill Flann Mór and Ailill Flann Bec

Ailill Flann Bec

Ailill Flann Bec, son of Fiachu Muillethan, was an Irish dynast belonging to the Deirgtine, the proto-historical ancestors of the historical Eóganachta dynasties of Munster. He was the father of Luigthech, also known as Lugaid, and thus the grandfather of Conall Corc...

.

Like his father Éogan Mór, grandfather Ailill Aulom

Ailill Aulom

In Irish traditional history Ailill Ollamh , son of Mug Nuadat, was a king of the southern half of Ireland. Sabia, daughter of Conn of the Hundred Battles, was his wife. He divided the kingdom between his sons Éogan, Cormac Cas, and Cian. Éogan founded the dynasty of the Eóganachta...

, and great-grandfather Mug Nuadat

Mug Nuadat

In Irish mythological history Mug Nuadat was a legendary, supposed King of Munster in the 2nd century AD. He was, according to later medieval tradition, a rival of the High King, Conn of the Hundred Battles and for a time after the year 123 was the de facto ruler of the southern half of Ireland...

, 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. This is apparent from the conception of Fiachu as described in the Cath Maige Mucrama.

While the career of Fiachu Muillethan may be entirely legendary, that of his supposed great-grandson Conall Corc

Conall Corc

Corc mac Luigthig, also called Conall Corc, Corc of Cashel, and Corc mac Láire, is the hero of Irish language tales which form part of the origin legend of the Eóganachta, a group of kindreds which traced their descent from Conall Corc and took their name from his ancestor Éogan Mór. The early...

, the true founder of the Eóganachta, may preserve an amount of historical fact.

As in many such cases, Muillethan and Fer Da Liach may have started out two different figures later thought to be the same, whether mythological or historical. 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.