McDermott's Castle, Castle Island, Lough Key, Co. Roscommon.
A castle has stood on this island from the 12th century. The original castle belonged to the Mac Diarmada clan who ruled the area from the 10th-16th century.
Legend tells the story of Una, the daughter of the McDermott chief, who was in love with a peasant named Thomas Costello. Her father prevented her from leaving the island to see him. Thomas swam to the island once too often to see her and drowned in the lake. Of course, the poor girl died of grief, and both her and the boy are supposed to be buried on the island, beneath two intertwined trees which grew over them.
In the summer months you can rent a rowing boat and row out to the island.