The views from the summit of the tower in St. Canice's Cathedral are excellent. I thin you have to ask the guy in the cathedral (priest or reverand...) for the keys - it's been many years since I climbed it on a school excursion into town.
The cathedral that stands today was built just over 700 years ago and is named after Kilkenny's patron saint, St. Canice. If you are not afraid of heights, it is great to climb the 30 meter tower for the great view. It is very steep, and you ascend 6 flights of steps which are basically ladders, and once at the top, there is room for about 8 people. It is a it dizzying, but well worth it on a nice day. There is a fee of about 2 Euros to climb the tower.