The thirteenth century Cathedral of St Canice stands overlooking the northern end of the city. The present building was preceded by an early monastic settlement, and subsequently a small Hiberno-Romanesque Church, which in turn was replaced by the present Cathedral. The round tower (31 m high) alone survives from the Monastery and was used as a belfry and a refuge in times of danger.

The Cathedral was begun by Hugh de Rous (1202-1218) in the Early Gothic style from the Angevin district in France, continued by his successors, and completed during the episcopacy of Geoffrey St. Leger (1260- 1286). Despite an eventful history, including the the Reformation in the mid-sixteenth century, wrecking by the Cromwellians in the mid-seventeenth century and restoration in the mid-eighteenth century, the Cathedral is unusually consistent in style. Its present appearance is due to a general restoration begun in 1866 by the Architect Sir Thomas Newenham Deane.

The building is Ireland's second largest mediaeval Cathedral. Although large, it was not a particularly ambitious church, and the unadorned architecture gives a wonderful sense of light and space.

The oldest part of the building is the chancel which was possibly completed prior to 1220, and contains a fine three light east window. This window once contained celebrated early mid-fourteenth century stained glass, which having survived the Reformation, was subject to a purchase bid by Rinucinni (the Papal Nuncio at the Confederation of Kilkenny) in 1645. The offer was refused and in 1651 the glass was smashed by Cromwell's troops.

The doorway is the finest surviving early Gothic doorway in Ireland and features many foliage cusps and two fine label-stops in the form of a human head. The master-mason for much of this late work is known as the Gowran Master since his handywork is most noticeable at Gowran church some of his characteristics appear at Thomastown. As a sculptor he was perhaps the most gifted craftsman in Ireland during the thirteenth century.