Fergus Dubdétach ("black-tooth") was, according to medieval Irish legend and historical tradition, a king of the Ulaid who was briefly High King of Ireland. He took the High Kingship after his predecessor, Lugaid mac Con, was expelled from Tara by Cormac mac Airt and killed in Munster by Cormac's poet Ferches mac Commáin. Fergus and his two brothers, Fergus Caisfhiachlach ("rough-tooth") and Fergus Foltlebair ("long-hair"), then expelled Cormac to Connacht and Fergus took the throne. He ruled for a year, before he was defeated by Cormac, with the assistance of Tadg mac Céin and Lugaid Láma, in the Battle of Crinna.
