Has anyone been to Clonmel? We'll be driving through it on our way to Kilkenny. Or is it better to drive through Durrow?

I should have said that we'll be driving from Cashel.

I have been to Clonmel, but I still don't get the question.
If you want to go from Cashel to Kilkenny, you don't go via Clonmel or Durrow - unless you have a map with only N roads in it ;-)
You'd leave Cashel in NE direction on R691 until you hit N76 near Kilkenny.

If you WANT to take the huge detour on national roads only, I would suggest to visit the castle and /or Swiss Cottage in Cahir, appr. 15km South of Cashel on N8/ M8. And then go East on N24 and finally NE on N76.
Clonmel is good for a walk in the city center, but the town does not have too many "must-see" sights (IMO!) for a huge detour.

I agree with Cowboy. Clonmel seems nice enough, but not someplace I'd go out of my way to get to in general.

There's the "magic road" near Clonmel!!! It is by Mahon Falls and if you stop your car on that road and put it in neutral, it will roll uphill! :=)