Nendrum Castle, Mahee Island, Killinchy, Co Down, BT23 6EP

The ruins of a tower house, probably built around 1570, stand at the northern edge of Mahee Island, facing the causeway from Reagh Island, approximately 2.5 miles northeast of Killinchy. The remains are three storeys high, constructed from split stone rubble and boulders. The original plan was likely rectangular, with surviving walls measuring roughly 13 meters by 6.5 meters. Significant portions of the northeast wall are missing, with only a corner section remaining on the right side. Much of the southwest wall survives and contains six openings of varying sizes, featuring splayed reveals. There are two openings at ground floor level, three on the first floor, and one on the second floor to the left, which appears to have been a doorway. Above one of the first-floor openings are niches that once held the second floor joists. At ground level, remnants of two rooms with barrel vaulting are visible; the room to the southwest extends through a wide doorway on the southwest side, though the northeast end has collapsed. The first floor appears to have contained a single chamber, and the remains of the stairway are located in the southeast corner, rising through the thickness of the southeast wall.

The castle is thought to have been built around 1570 by Thomas Browne, an English soldier who leased Mahee Island from Bishop Merriman. In 1572, Captain Browne requested resources from Lord Burghley to cover the cost of construction on the island, reported to be 400 marks (£266-4-0). The structure is considered a monument.