An Ulster warrior who fought with Medb and Ailill against his own people in the Táin Bó Cuailnge [Cattle Raid of Cooley]. He came to the aid of Cúchulainn when that hero was about to be drowned by the Clan Cailitin. This Fiachu appears to be identical with the character of the same name in the Fenian Cycle. He inherited a magical spear from Cumall, the father of Fionn mac Cumhaill, which when placed against the forehead filled a man with strength and battle fury. Eventually Fiachu ceded the spear to Fionn.