St. Mary's was originally a neo-Gothic church designed by J. J. McCarthy. The foundation stone was laid in 1862. It originally had a nave and aisles separated by arcades, supported on columns capped by octagonal tops. The arcades were demolished in one of the most radical reordering schemes to have been executed in Ireland. The project also saw the demolition of the exterior walls to below the original clerstory level, and, most notably, of the attic and upper ranges of the west elevation.