Too much in a day?

It just depends on how long you stay at each. Probably not, but maybe.

I'd suggest restricting yourselves to either Powerscourt or Glendalough but probably not both unless you really want to run yourselves ragged that day.
One way to make this work would be to collect your car at the airport as early as you can reasonably manage - the airport concessions open as early as 5 AM whereas the locations in the city typically don't open until around 8:30 or thereabouts, so you could get a very early start indeed if you so choose.
With an early enough start you could stop at Glendalough well before the tour buses (and the crowds) start to roll in at 9:30, which is when the visitor center opens. The monastic site itself is always open so by arriving early you'd have the place all to yourselves ... a truly magical experience as compared to later in the day. Makes for some great photos too.
Drive times between Glendalough, Kilkenny, Cashel, and on to Kinsale aren't excessive so you could make a pleasant, meandering day of it by just taking your time and not rushing it.
You'll be pretty tired by the time you pull into Kinsale that afternoon, but happily so I think after having seen some of the highlights of southeastern Ireland. At any rate you'll have plenty to talk about over a pint and dinner at Fishy Fishy - one of the highlights in Kinsale.
Or, for a special treat walk the shoreline path towards Charles Fort and stop for a pub dinner at Bulman's in Summercove - one of the area's better kept secrets.

Thanks so very much!

Have you checked on distances? You are talking about well over 200 miles drive which is going to take 6+ hour to drive PLUS any stops on the way.... Don't underestimate the time needed to find somewhere to park in places like Kilkenny or Cashel and then walk to the attraction. That can easily add another 30 minutes to your drive times.

Distances may not look that far on a map compared with USA but will take a lot longer to cover than you might expect. If using google maps then recommended advice is top add 25% onto their times plus time for stops. Ireland is the kind of place you need to slow down and enjoy!

We travelled from Dublin to Cork in a day with stops at Glendalough and Rock of Cashel. Afterwards we felt that we hadn't allowed enough time for either and regretted this as there was so much to see and enjoy in both places.

It is doable as long as you are happy to view places like Kilkenny Castle and cathedral and Rock of Cashel from the outside.

Each of the places you've listed deserve 1-2 hours to visit.

I’m assuming you will be traveling in the summer when you have the most daylight. That will help but I personally would not try to do all that in one day. I have been to Glendalough, Powerscourt and the Rock of Cashel twice. At each place I felt rushed even though I was not on a timetable.

Regarding Kinsale, if you plan on dining at the popular restaurants you will need dinner reservations at least a day in advance (unless you plan on eating at 4:30 or 10:00). Bulman’s is a great lunch spot. For splurging I suggest Finn’s Table.

Thanks again for your suggestions. Going to Kilkenny via Wicklow sights is probably too much I agree.
I actually tried to make reservations at Fishy Fishy online and have not received a reply. As far as Kilkenny - was just thinking of checking it out and maybe having some lunch as per RS book...so not touring castle or anything.
Also first day on the road don’t want to get too crazy. There’s just so much to see and it’s hard to leave some things out.
Thanks again for all your helpful comments!

We did the same thing, spent a few days in Dublin on arrival then returned to the airport to pick up our car to head south. Try to get to the car rental facility as early as possible, it took us about an hour just to get our car. Also, make sure your cab driver takes you to the car rental facility, not the airport itself. We ended up having to take a shuttle from the airport to the car rental facility which is off site. Getting on the highway from there was easy and the road was very nice.

We probably spent 3 hours in Glendalough visiting the monastic city and walking to the waterfall.