The Church of Ireland Cathedral (said to be the smallest Cathedral in Europe) stands on the site of the early 13th century medieval Cathedral, which was built up by Bishop John St. John at the same time as the building of the Castle. John St. John was the first Norman bishop of Ferns, d 1243. It was situated within the bounds of the monastery founded by St. Aidan in the 6th century. The Cathedral was largely destroyed by the O’Byrnes of Wicklow in 1575. Ordered by Queen Elizabeth 1 to rebuild the Cathedral, in 1577 they reinstated a small part of the nave. On the inside of the present building, some internal pillars of the original structure can be seen built into the north and south walls. This area was again rebuilt in the early 1800s, as commemorated in the date 1817 over the door.