Declán mac Eircc is an early Irish saint remembered for having converted the Déisi in the late 5th century and for having founded the monastery of Ardmore. Declán preceded St. Patrick in bringing Christianity to Ireland.

The Cathedral's chancel dates from the 9th century, with additional construction occurring throughout the centuries. The Cathedral contains 8 medieval grave slabs and 2 Ogham stones, whose markings are an alphabet used in 5th century Ireland. To the east of the Cathedral is St. Declan's Oratory, a small 8th century building believed to be the burial place of St. Declan. The Round Tower dates from the 12th century.