St. Mary's Church stood further east of St. Patrick's, which it resembled in design, but was more beautiful in detail. The capitals of the pillars, the mouldings of the arches, the tracery of the widows being more ornamental than those of either Selskar or St. Patrick's. It was, on the testimony of the late Father Corrin, the last Parochial Church in use from the time of the Reformation until the building of the new Churches in 1858.

Dr. Nicholas French, Bishop of Ferns, was the last Parish Priest of St. Mary's from 1638 until he was driven into exile in 1651. His residence was the large house at the top of Peter Street (now divided into two houses). At the rear of this house is the traditionally known "Bishop's Garden" extending down to the walls of St. Mary's Churchyard. The doorways leading from the garden to the Church precincts, though built up, are still clearly discernible today.