Located on a rocky outcrop in Southern Tipperary, Cashel is a nice place to stop on the way down to the South West. It sits up and dominates the landscape; it is pretty evident why this site was chosen for a fortification.
There are many sites like this in Ireland boasting remains of a castle, a round tower, a ruined cathedral and a high cross. Cashel is one of the more impressive sites in the country and is well worth the time to see if you are heading down to the South; it is on the Dublin - Cork bus route, providing an ideal break in the journey.
Whilst the remains are impressive, being one of the finest examples of Celtic art from the 12th-13th centuries, I'm not sure they would make it onto the list. Ireland does not seem as intent as other countries to use the list as a means to attract tourists and as such these sort of sites will not be promoted as they would elsewhere in the world. However I think this is a very impressive site and well worth a stop.