Ferdiad (also Fer Diad, Ferdia, Fear Diadh), son of Damán, son of Dáire, of the Fir Domnann, is a warrior of Connacht in the Ulster Cycle of Irish mythology. In the Táin Bó Cúailnge, Ferdiad finds himself on opposite sides to his best friend and foster-brother Cúchulainn, with whom he had trained in arms under the renowned warrior woman Scáthach. He and Cúchulainn are equal in all martial feats, with two exceptions: the Gáe Bulg, a barbed spear which Scáthach has taught only Cúchulainn to use; and Ferdiad's horny skin, which no weapon can pierce.
When Ailill and Medb, king and queen of Connacht, invade Ulster to steal the bull Donn Cúailnge, their progress is held up by Cúchulainn, who demands single combat. After Cúchulainn has defeated a series of Connacht champions, Medb sends for Ferdiad, but he only agrees to fight Cúchulainn after Findabair, Ailill and Medb's daughter, has seductively plied him with alcohol, and Medb has variously bribed, shamed and goaded him to do so. They fight in the ford for three days, until Ferdiad gets the upper hand. Cúchulainn calls to his charioteer for the Gáe Bolga, which he floats down the river to him. Cúchulainn throws a light spear at Ferdiad's chest, causing him to raise his shield, and then picks up the Gáe Bolga between his toes and thrusts it through his anus upon which the barbs spread throughout his body, killing him. The ford where he died is named Áth Fhirdiad (Ferdiad's ford, modern Baile Átha Fhirdhia or Ardee, County Louth) after him.
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.Ferdiad means "man/warrior of the pair" (Diad related to dïas, "two persons").