The ruins of the Abbey of St Mary, stand at the northern end of Bardsey Island, south-west of the Lleyn Peninsula, in Gwynedd, Wales.

By the end of the Middle Ages the abbey had declined in importance and, following the Dissolution of the monasteries, fell into ruin.

[1] By the High Middle Ages the island was reputedly the burial place of some 20,000 saints.

Richard Haslam, Julian Orbach and Adam Voelcker, in their 2009 edition Gywnedd, in the Buildings of Wales series, note its setting within an walled enclosure.

[2] The Royal Commission on the Ancient and Historical Monuments of Wales records the dimensions of the "two-stage" tower as 5.8m square with 1.0m thick walls and standing 8.0m high.