In the Chapter House of the ruined church are six voussoirs with human heads, found in 1865 during demolition of the 17thc. tower, where they had been reused as building stones.

A monastery was founded at the site by St Finbar in the 6thc. Abbots are recorded from the late 7th to the 12thc. Bishops are recorded in 876, 891, 958, 1036, 1057 and 1096. A large territory was given to the Bishop of Cork at the synod of Rathbreasail, but after 1119 the territory was reduced by the emergence of the dioceses of Cloyne and Ross. In c.1134 or before his death in 1138, Cormac MacCarthaig founded an Augustinian monastery, known as Gill Abbey, about 400 m W of the cathedral at Cork. The abbey was largely destroyed in the 17thc., and the remaining buildings were demolished in 1745.

The voussoirs appear to come from the outer order of an arch. Bradley and King point out the variations in thickness of the edge-mouldings, indicating the original positions of some of the voussoirs on the left or right of an arch. The extending chins would have overlapped the next inside order. The heads are similar to those at Cormac's Chapel, Cashel, especially those in the chancel arch and below the chancel vault, and they appear to be the products of the same school. The carving is of high quality, with finer detail and finish than Cashel. Bradley and King suggest that the voussoirs might come from Cormac MacCarthaig's church at Gill Abbey. The close connections with Cashel and the common patronage of Cormac MacCarthaig indicate that Cormac’s Chapel was one of a group of stylistically related buildings, rather than the unique source of Romanesque influences in Ireland.