[Ir., broad crown, flat head].
Legendary ancestor of the Eóganacht kings of Munster, thought to be a contemporary of the illustrious Cormac mac Airt. A widely known story explains the unusual circumstances of his birth and conception. The druid Dil Maccu Crecga wished to have grandchildren more celebrated than himself, and so he invited Eógan (3) of Munster to lie with his daughter Moncha. Shortly after, Eógan (3) died in battle. When the time for the delivery came, Moncha went with Dil to the River Suir in Tipperary, where the druid-father announced that any child born on this day would become the chief jester of Ireland; if his birth were delayed a day, he would become a most powerful king. Moncha sat on a stone in the river, suffering great pain, but the child's birth was delayed one day. In the delay the baby's head was flattened against the stone, giving rise to his usual sobriquet, Muillethan, ‘broadcrowned or flat head’. Moncha perished in the difficult delivery, so that Fiachu lost both parents at birth, a fate he shares with Cormac mac Airt; yet only Fiachu was known as fer dá líach [the man of two sorrows]. More importantly, Fiachu was the first member of the Eóganacht dynasty, and was cited in many genealogies. Many other stories link the Munster king with Cormac mac Airt. Fiachu Muillethan was first allied with Cormac mac Airt in regaining Tara from Fiachu Araide, but later Fiachu Muillethan defeated Cormac when he invaded Munster. This story is told in Forbais Dromma Damgaire [The Siege of Knocklong], where Cormac bears the patronymic ua Cuinn. In reward for assistance in defeating Cormac, Fiachu granted Mug Ruith an extensive tract of land, Mag Féne, in what is now north Co. Cork, extending north from the Nagles mountains to the Ballyhoura hills.