Foster brother of the legendary Irish hero Cú Chulainn .
Táin Bó Cúailgne the longest tale in the Ulster cycle , deals with a conflict between Ulster and Connaught over the famous brown bull of Cooley. Joining the forces of Connaught in an expedition to seize the bull, Fer Díad engages in a three-day battle with Cú Chulainn and is defeated.