In 630, it was the location of a synod, where St Laisrén convinced a group of Irish bishops to relinquish the Celtic method of calculating the date of Easter for the Roman one.

The present building was begun by Donat O'Kelly or Donatus, Bishop of Leighlin from c.1152 to 1181 and was finished by the end of the 13th century with the completion of the nave and choir.

The two transepts, a tower, and a chapel attached to the choir were added in the 16th century, the latter two occurred in the time of Matthew Sanders, Bishop of Leghlin from 1529 to 1549.

The sedilia has four bays, the last a level above its fellows, and is flanked by a pair of trefoiled heads, dating from the late 13th century.

There is also evidence of haphazard additions: a 17th-century buttress blocks a 13th-century cancel window and there are blocked-up doors with no known purpose.