Planning to go to Glendalough and Rock of Cashel as stops on drive from Dublin to Kinsale. Is it too much to stop at Powerscourt as well? Is it worth the time it would take?
Including all 3 stops on the way to Kinsale from Dublin would make for a very long (and potentially exhausting) day, especially if you're newly arrived and possibly still a bit hung over from jet lag. It's doable with an early start, and a steely constitution, but know what you'd be getting yourself into.
Might suggest doing Powerscourt or Glendalough, but not both. Your call as to which of the experiences is most appealing to you. As I recall Powerscourt doesn't open until around 9:30. The visitor center at Glendalough also opens around 9:30, but the monastic settlement itself is open any time you care to arrive, meaning that with an early start you could visit the site ( having it pretty much all to yourself) before the tour buses start to roll in and then be on your way.
Cashel will be right on your way on the drive down to Kinsale. While the Rock itself is the big attraction (rightfully so) if you'd like to stretch your legs a bit and roam a little further afield you could walk down the hill to the ruins of Hore Abbey...and maybe grab a picnic lunch to eat there while looking back up at the Rock - it's a very evocative place in its own right.
A more relaxed schedule would get you into Kinsale still perky enough to explore the town... maybe walk down the coastal path to the village of Summercove for a pint of Murphy's at Bulmer's pub.
Glendalough can take a couple of hours, as can Rock of Cashel. It would help to know if this is on your arrival day from the US or not. I have not been to Powerscourt. Whether or not it is worth the stop could depend on what time of year you will be traveling. My last trip to that area was in late February, so it didn't make sense to stop at gardens that wouldn't be in bloom at that time.