We will be touring Ireland and I'm not sure we will have time for both Kilkenny and Cashel. We will be coming from Dublin by public transport. I would consider a private tour but I can't find any tours private or group tours that combine both. Thoughts? Thank you.
If memory serves correctly, Kilkenny has a castle and a church that is made of black stone. Other than that I don't remember being too impressed. But we were just there one evening and part of the next day. Cashel, however, was very impressive. Perhaps you can google each of them and get a good idea of what each has to offer.
Unless you really want to shop, I found both of these locations were worth a half-day. Kilkenny has some medieval charm - it's like "York lite." For Cashel, after admiring the view from the nearby ruined abbey, go up and wander around the rock. The town is geared to tourism, although we did have an outstanding meal at a restaurant that was in a re-purposed church. I think the restaurant was named Ian's. The beauty of Ireland is its scenic coasts - I wouldn't give up too much vacation time for the interior towns if I could be spending it closer to the coast line!
To give a different point of view:
We were in Ireland last May and went to both places and I would recommend skipping the Rock of Cashel because it is mostly just a pile rumble, the day we were there it was very underwhelming, in fact the only thing we got out of our visit was a beautiful view of the Irish countryside. Kilkenny was interesting, we stayed at Butler House (great B&B with amazing breakfast) and toured the castle, enjoyed the Old Town area for shops & pubs, the church at the other end of town is wonderful and the bell tower will give you a even better view than you get at the Rock. We spent two nights in Kilkenny and truly enjoyed it where as once you get to the Rock of Cashel it seems like a tourist trap & the town around it is very touristy oriented.