Powerscourt Estate Gardens in Wicklow Mountains in Ireland

I've been to the Powerscourt gardens twice, once in late spring and once in early fall and enjoyed them both times. Unfortunately I have yet to make it to the waterfall which is about a 15 minute drive away. I can't tell you whether either is a "must see", I suppose it depends on whether you like gardens. You can easily spend a couple of hours at the gardens, especially if you dawdle like me, have a snack in the cafe or shop at the ever-present Avoca shops. I think it should be perfectly reasonable to stop at the gardens on your way to Kilkenny.

By the way, there's a relatively new distillery at Powerscourt. We had a nice tour and tasting there. They were kind enough to give our driver whiskey flavored ice cream.

I'm not sure I'd call it a must-see but it's a beautiful place and an interesting half day. Yes, Kilkenny is a reasonable drive from there, about 2 hrs. Unless you're in a hurry, I'd travel down the coast and then cut inland. Once we stopped in a little town to walk down to the water (millions of tiny smooth pebbles) and get a pub lunch. Or you could visit Glendalough in the Wicklow Mountains National Park. It's another small sight but we really enjoyed the scenery in the area.

Marie, if you post this in the destination section (rather than trip reports) you'll get more responses, I'm sure.

I wouldn't categorize Powerscourt as a must see unless you're really, really into gardens and have plenty of extra time on your hands.
The more evocative site is several miles away at Glendalough which, for our money, truly qualifies as a "must see".

Agree with Robert Glendalough, over Powerscourt.

Then if not rushing to get to Kilkenny ( and IMHO Eire is ALL about NOT rushing ) meander.

Slow down. Watch for dog walkers, sheep, lorries, traps, bicyclists, backpackers. Stop at a rural pub for lunch. Appreciate the beauty of the country and the Irish.

Slainte