In Irish mythology, Ferdiad (also Fer Diad, Ferdia) was the son of Daire (or son of Damáin son of Daire) and the champion of the men of Domnand (Fir Domnann) from Irrus Domnann in Connacht. Along with the Fir Bolg, the Fir Domnann were among the early inhabitants of Ireland before the coming of the Tuatha de Danaan or the Milesians. "Firdiad" means "man/warrior of the pair." ("Diad" related to dïas, or "two persons.") [1] The mythology of pre-Christian Ireland did not entirely survive the conversion to Christianity, but much of it was preserved, shorn of its religious meanings, in medieval Irish literature, which represents the most extensive and best preserved of all the branches of Celtic mythology. ... Fir Domnann was an ancient Irish tribe or people located in the west of Connacht, in what is now the Erris Peninsula in County Mayo. ... In Irish mythology the Fir Bolg (Fir Bholg, Firbolg, men of Builg or men of bags, or possibly men with spears, bolg meaning spear - and let us not forget the modern Irish word bolg belly (originally bag)) were one of the races that inhabited the island of Ireland prior to... The Tuatha Dé Danann (peoples of the goddess Danu) were the fifth group of inhabitants of Ireland, according to the Lebor Gabála Érenn (Book of Invasions) tradition. ... In Irish mythology the Milesians or Sons of Míl Espáine were the final inhabitants of Ireland, representing the Goidelic Celts. ...

More famously, Ferdiad was Cúchulainn's best friend and foster-brother. He trained with him for years in Scotland under the famed She-Warrior Scáthach, and they were considered equally extraordinary warriors—though Cúchulainn had the Gae Bulg, Ferdiad possessed a coat of horn-like armor that no ordinary weapon could pierce. He was persuaded to fight against Cúchulainn by Queen Medb in the Táin Bó Cúailnge as a last resort, Cúchulainn having already slain many other champions in combat. Though Ferdiad initially refused to fight his foster-brother, Medb threatened him with vicious satires and offered him her daughter Findabair if he was successful. Finally and reluctantly, he agreed to go to battle. Young Cúchulainn, 1912 illustration by Stephen Reid. ... Scáthach (shadowy) is the female warrior who trains Cúchulainn in the arts of war in the Ulster Cycle of Irish mythology. ... The Gáe Bulg (also Gáe Bulga, Gáe Bolg, Gáe Bolga, meaning notched spear, belly spear, bellows-dart, or possibly lightning spear) was the spear of Cúchulainn in the Ulster Cycle of Irish mythology. ... (, Medb, Medhbh, Meabh, Maeve, Maev) is queen of Connacht in the Ulster Cycle of Irish mythology. ... Táin Bó Cúailnge (the driving-off of cows of Cooley, more usually rendered The Cattle Raid of Cooley or The Táin) is the central tale in the Ulster Cycle, one of the four great cycles that make up the surviving corpus of Irish mythology. ... In Irish mythology, Findabair or Finnabair (whose name means fair eyebrows) was the daughter of Ailill and Medb of Connacht. ...

After three days of battle at a river ford, Ferdiad was killed when Cúchulainn used his magical spear the Gae Bulg, which once thrown could not be stopped.

Scholars believe that the fight between Cuchulainn and Ferdiad is a late addition to the Táin, originating not earlier than the eleventh century and drawing on earlier episodes in the story.