A cathedral was built here in the 13th century probably by John Saint John, the first English bishop of Ferns, d. 1243. It stood within the bounds of the monastery founded in the 6th century by Saint Aidan also known as St Máedhóg (who died in 624), who was reputed to have been a pupil of St David of Wales. Sometime after his death he was succeeded by St Moling who had already founded a church at St Mullins, Co. Carlow.

This large cruciform cathedral was burnt down by the O Byrnes of Wicklow in 1575. They were ordered to rebuild it in 1577, which they did, though crudely, to approximately the size it is today. The east end wall with its lancet and vesica shaped windows was rebuilt from part of the original 13th century building. Two ancient vaults run beneath the chancel. In 1817 the cathedral was enlarged and the present tower and chapter house added. On the inside the pillars of an earlier structure, walled up in 1577, can be seen in the north and south walls.

Diarmuid MacMurrough is most famous for having invited the Normans to Ireland in 1169 A.D. Diarmuid's grave is in the cemetery, a broken granite shaft of a High Cross with a distinctive fret pattern decoration is reputed to mark the grave of Diarmuid MacMurrough (d.1171), Originally a High Cross, during the Cromwellian invasion the Cross was destroyed and only the shaft remains.

The remains of a 13th century cathedral, part of which is incorporated into the present Church of Ireland Church, St. Peter's, a small nave and chancel church, and some High Crosses and parts of crosses, also stand in the cathedral grounds.