We visited St Mullins last Oct on a day-trip outing from Kilkenny. Not mentioned in your link, there is also a very famous Holy Well located nearby also. The ruins was almost on par with Glendalouch and have not yet visited Clonmacnoise, tho it is on my short list! A bonus is a couple small rural villages nearby with quaint pubs, that deserve a visit! I think Carlow county as a whole is under rated and unvisited. Highly recommended!
Dan