 Their ancestry is cited as being from Tuathal, a son of Daimíne, making them part of the Síl nDaimini.The Síl Fiachra Cassán, descend from Fiachra Cassán, a son of Colla Fochríth.Airthir (barony of Lower and Upper Orior), meaning 'east', was one of the main branches of the Síl Fiachra Cassán until the 8th century when it split into the main septs of the Uí Nialláin, the Uí Bressail, and the Uí Echdach.