A ruined nave (16.4 m x 8.1 m) and chancel (6.35 m x 6.67 m) church with a W tower and a sacristy on the N side of the church. Built of rubble masonry. The remains comprise the N walls of the chancel and most of the nave, the walls of the sacristy, and the W tower. Only the foundations remain of the S walls. The springing of a barrel vault with transverse arches remains on the N side of chancel, which evidently had an upper chamber. The sacristy to the N of the chancel is reached by a doorway from the E end of the nave and this also has the remains of the springing of a barrel vault on the S side. Stairs to the upper chambers of the chancel and sacristy rise from the SE corner of the sacristy. The W tower is square at the bottom and round at the top and is set at the N side of the W front and entered by a doorway from the W end of the nave.

A monastery was founded at Ferns in the 7thc. by St Mogue. The diocese of Ferns was established in 1111 at the synod of Rathbreasail. The Augustinian abbey of St Mary was founded c.1158 (or by 1162?) by Diarmait Mac Murchada Uí Chennselaig, King of Leinster. Ferns was the capital of Diarmait's territory. He was buried in St Mary's following his death in 1171.

The barrel vaulted chancel with an upper chamber is similar to the chancel of St Saviour's, Glendalough, and the nave of Cormac's Chapel, Cashel. Henry dates the building 1140-58, and O'Keeffe also suggests a date in the 1140's or early 1150's based on the parallels with St Saviour's, Glendalough.