I would say Kilkenney (for the night life) or Cahir (for the relaxation). I would definitely pick up Cahir BEFORE Wexford (Cashel to Cahir to Clonmel to Kilkenny on the scenic N76), then take the R700 to Wexford.

We just returned from 2 weeks in Ireland, visiting many of the places of your itinerary. I don't know. Maybe it's just that we're real "panty waists" but we found the driving VERY difficult. If I had it to do all over again, I would probably opt for an automatic rather than the stick shift. Doing so would make one less thing to worry about. As for making reservations, I did and I REGRET it. Ours were prepaid for via a travel agent and therefore could not be cancelled. They were in all the wrong places! Don't miss the Rock of Cashel. While there, go to the Bro Bru show in the evening. Lots of traditional Irish music and dancing. Cheers!

mkdiebold, When you say your reservations were in 'all the wrong places,' where were they, and why do you feel they were in the wrong places? Now that you've been there, what do you feel would have been a better itinerary?

mkdiebold was there the end of June, pretty close to high season. That would tend to make the traffic a lot worse than you should expect to see during September, but you will likewise have fewer hours of sunlight. We've been six times now and I do all the driving. Don't do much night time traveling, personally (as seeing the scenery is at LEAST half the charm!) and I've driven everything from 8 passenger Mini-buses (TWICE), to a compact Alfa-Romeo 146 and vehicle in between (All of them manual shift.). If you are skittish about the driving, be sure to rent no larger a vehicle than you need -- bigger isn't safer in Ireland -- it makes the driving more difficult.
I have this funny story about that -- we were checking into the Jurys Inn in Galway (6 of us, in '99), and I asked the desk clerk for a voucher/discount for the adjoining parking garage. When I told him than I had a VW Carravelle that I had parked there (on the 4th or 5th level), he was AMAZED and told the others that they each needed to buy me a drink as a reward. He said he only drove a small Mazda 323 hatchback, and it TERRIFIED HIM to manuever IT through the garage!
The driving is what you make it. Don't be in a hurry and DON'T RUSH and you'll do fine.