Mahee Castle, also known as Nendrum Castle, is a small ruined tower house near Nendrum Monastery on Mahee Island in Strangford Lough, County Down, Northern Ireland. It was built in 1570 by Captain Thomas Browne.[2][3] It was abandoned by the early 17th century, and fell into disrepair.[4] In 1923, H.C. Lawlor and the Belfast Natural History and Philosophical Society partly renovated the tower house to avoid further erosion and built a buttress wall to support the northwest corner of the tower.