Nendrum Monastery was a Christian monastery on Mahee Island in Strangford Lough, County Down, Northern Ireland (Irish: Loch Cuan, Contae an Dúin, Éire). This is said to have been founded in the fifth century by St Machaoi. The remains on the site include three concentric dry stone enclosures. The central enclosure has the remains of a round tower, a graveyard and ruined church and sundial. The sundial is thought to date from about 900AD. The second enclosure has the remains of workshop and burials. Items from the site are kept at the University of Ulster, including slate tablets with Celtic designs, a stone with inscriptions in runes and a bell.