The ruin looks like a fortified church, and it is, but it began as a friary, became a contested piece of suppressed property, was misidentified in official records as Dominican when it was Augustinian, and outlasted its own dissolution by several centuries of quiet stubbornness. Even the well inside the enclosure walls, dedicated to St Nicholas and the site of a pattern held every 6th December, has vanished into the ground, its location now traceable only by geophysical survey.

The church was founded by the Kavanaghs for Augustinian Friars around 1317, a grant of land establishing the community at this medieval port town on the Barrow estuary. In 1385, Nicholas FitzNicholas enlarged the building, and it is likely that the fortifications were added around the same time, giving the complex its unusual hybrid character, part religious house, part defensible compound. The enclosure, known as a bawn, a walled defensible yard of the kind more often associated with tower houses and plantation settlements, measures roughly 67 metres by 60 metres, with a corner tower at the north-west angle and a gatehouse fitted with a portcullis slot attached to the west end of the nave. When the Dissolution came in 1540, the house was recorded, incorrectly, as Dominican, and had little property to speak of beyond a few plots at Clonmines. The friars were expelled in 1544, but they did not leave the area entirely; they continued to serve locally, and by 1773 had settled on a small farm at Grantstown in Barrystown townland, about 2.4 kilometres to the east. A chapel there was eventually replaced by a new church and abbey in 1830.

The original church ruin has survived in a remarkably complete state, with the notable exception of the north and west walls of the nave. The chancel retains three damaged three-light windows with ogee heads, a pointed arch typical of late medieval Irish Gothic work, curving to a gentle S-shape at the top. An aumbry, a small wall recess used to store liturgical vessels, remains in the south wall alongside the remains of a triple sedile, the stone seating used by clergy during Mass. A tower inserted at the east end of the nave blocks what was once a three-light window in the north wall, and its stepped parapet still carries lookout platforms at two corners. On the exterior of the north church wall, two pairs of corbels mark where the lean-to roof of the cloister garth once met the stonework, the cloister and associated buildings having stood on the north side of the church. A Francis Grose illustration from 1791 recorded a large decorated window in the now-destroyed west wall of the nave, the only record of what was presumably the most elaborate feature of the original building.