Kells was the medieval capital of Kilkenny. This Augustinian Priory was founded in 1193 by Geoffrey FitzRobert, brother-in-law to Strongbow. The priory is purported to be the largest monastic site in Europe.
This is a site that is not to be missed, though it sadly seems to not get much attention. The benefit of this is you will likely have the entire place to yourself if you visit. Another benefit - it's FREE!