St Mary's Priory was established as a daughter house of the Tironensian St Dogmael's Abbey (NPRN 94164). It was founded in 1113, probably on the site of an earlier religious establishement. The priory was dissolved in 1536, after which the conventual buildings were incorporated into a secular residence.

The priory is Grade I listed, and is of rubble stone construction with a slate roof. The buildings are grouped around a small courtyard. On the north side is a domestic range. The gatehouse is on the western side and there is a two storey tower on the eastern. The church, St Illitud's, stands to the south side, and consists of nave, chancel and west tower with a spire. An early medieval ogam and roman-letter inscribed and cross-carved stone, Caldy 1 (NPRN 415019) is set against the south wall of the nave. There is a cusped thirteenth century piscina in the south wall of the chancel. and there is a gatehouse and guesthouse in the western range. The refectory is to the north and the dormitory to the east. The Prior's Tower is at the north-east angle. The church dates from the thirteenth century. There were additions in the fourteenth century and alterations in the fifteenth and sixteenth. The church quire had been remodelled as a two storey farmhouse by 1800, and the complex later became the anciliary building to a new house. It was restored in the nineteenth century. A second possible medieval monastic establishment stands on St Margaret's Isle (NPRN 30205).

RCAHMW, 22 May 2013