This is the best example in Northern Ireland of a pre Norman ecclesiastical enclosure with its buildings. Nendrum is associated with St Mochaoi who died at the end of the 5th century, and is linked with St Patrick in a much later source. Notices of Nendrum clergy, including three bishops, begin in the 7th century and the excavation of a sophisticated tide mill on the shore near the site has shown that the mill was active in the early 7th century, which underlines the early importance of Nendrum. Annal references continue until a fire in 976, perhaps a Viking raid, when the head of the church (erenagh) was burned in his house. In the late 12th century a small Benedictine monastic cell was founded on the site, but by 1302–1306 this was the parish church, abandoned for Tullynakill on the mainland in the 15th century.
H.C. Lawlor excavated the site from 1922 to 1924, when enclosure walls, church and round tower were restored. The glacial hill is crowned with three concentric walled enclosures, irregularly oval in plan. Little is known of the outer cashel, only partly in state care. In the middle cashel on the south-west side are circular platforms for huts, which excavation suggested were craft workshops, and a rectangular building known as the ‘schoolhouse’, also a workshop. In the inner cashel were the most important buildings, including the church with its graveyard and the base of a round tower north-west of the church. The west wall of the church was rebuilt in the 1920s, incorporating a reconstructed sundial at the south-west corner. Finds from the 1920s excavation, including the well-known bell of Nendrum, are in the Ulster Museum and Down Museum at Downpatrick. Underwater archaeological work in the 1990s showed that there is a stone jetty south-east of the enclosure, now inundated in the lough, and medieval pottery was found close by.
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