The O'Dempseys are a branch of the O'Connors of Offaly and were a powerful sept in Clanmalier on the borders of Leix and Offaly. Their chief was one of the few who won a battle against Strongbow, leader of the Anglo-Norman invaders, in 1172. Later, as supporters of James II, they were ruined as a sept and dispossessed of their lands following the Jacobite defeat at the Battle of the Boyne. Dermot O'Dempsey (d. 1193), chief of the sept, founded the Cistercian Abbey at Monasterevan.
