A church has existed on the site of St. Mary’s since at least 1374 AD.
The date of its foundation is not known, but in 1374 Alexander, Bishop of Ossory, confirmed the Church of Castlecomer to the Prior and Canons of St. John’s Abbey, Kilkenny. In 1637, Christopher Wandesforde begun the rebuilding of Castlecomer to include the church currently standing. Along with most other buildings in Castlecomer, St. Mary’s was badly damaged in the 1798 rebellion. Thankfully, Lady Ann Wandesforde restored St. Mary’s Church as seen today.