Nendrum monastic site, Mahee Island, Killinchy, Co Down, BT23 6EP

Nendrum monastic site contains the ruins of an early Christian ecclesiastical settlement located at the southwest end of Mahee Island, an L-shaped island near the western shore of Strangford Lough in County Down. The island is now approached via a causeway.

The site comprises three concentric enclosures built of dry stone walling. At the north side of the outer cashel is a small circular stone-lined hollow, apparently once a kiln. The middle cashel contains foundation remains of a small rectangular building on the west side, which may have been a school, along with foundation outlines of other small buildings. A small timber-built twentieth-century building now serves as a visitor and display centre on the south side of this middle section. The inner cashel contains the most substantial remains: a small rectangular church and a round tower. Remnants of a sundial have been assembled and attached to the church walls, along with an inscribed panel commemorating the 1922–24 excavations. The inner cashel walls have several entrances.

The site is named after St Mochai, a missionary bishop said to have been converted to Christianity by St Patrick in the fifth century and believed to have been based here. Records of the monastery's foundation begin only in the seventh century, revealing a vibrant and apparently substantial community. The monastery was attacked in 976 when the abbot was burned in his own house, though by whom remains unknown. Following the Norman Invasion in the late twelfth century, John de Courcy established the Benedictine Rule. Nendrum was recorded as a parish in Papal taxation records of 1306, but by the mid-fifteenth century parish worship had moved to the mainland church of Tullynakill and the site was abandoned.

The site was rediscovered in 1844 when William Reeves (later Bishop Reeves) recognised a 'limekiln' as a round tower with nearby church ruins. The 1834 Ordnance Survey map marks the site with the cryptic phrase 'Old town', suggesting some local knowledge of the once-populous settlement remained. Large-scale excavations were carried out in 1922–24 by H.C. Lawlor and the Belfast Natural History and Philosophical Society. Unfortunately this work was poorly recorded, and the accuracy of the site's 'restoration' remains unclear and has presented problems of interpretation since. The form of much of the structures visible today results from these extensive excavations, with many buildings and walls 'reassembled'. Much of the site is now in the care of the Department of the Environment.