Castleisland was the centre of Desmond power in Kerry. The town got its name; Castle of the Island of Kerry from a castle built in 1226 by Geoffrey Maurice (or de Marisco), who was the Lord Justice of Ireland during the reign of King Henry III. The island was created by turning the waters of the River Maine into a moat around the castle. Sometime in the 120 years after its construction the castle was taken by the forces of the Earl of Desmond. It is known that in 1345 the castle was being held for the Earl of Desmond by Sir Eustace de la Poer and other knights when it was captured by Sir Ralph Ufford, Lord Justice of Ireland. Sir Eustace and the other knights were captured and executed. Little is known of the further history of the castle, few ruins are left of it today. The main ruin is de Marisco tower, located behind some private houses at the western end of the town, on the Killarney Road.