realistic to visit Powerscourt, wicklow, and glendalough then spend night in Kilkenny?

Yes, that’s doable. Keep in mind that “Wicklow” is an entire county/region and not a destination in itself like the other two. You’ll be driving through County Wicklow as you travel from Dublin to Glendalough and Powerscourt, so you’ll see it all. Just keep an eye on the time. You can easily spend several hours at both Glendalough and Powerscourt, plus there’s driving time. Have fun!

Yes, we did that a few years ago. A lot will depend on when you get away from Dublin and how much time you want to spend at Powerscourt and Glendalough. As I recall, we checked into our BnB and then had a fairly late dinner.

With an early start you could exit the M50 at the Old Military Road (the R115) and drive south to Glendalough via the Sally Gap thru Wicklow Mtns. National Park. The road was built by the Brits around 1800 to allow their patrols to chase Irish rebels after the 1798 rising. It's quite a picturesque drive on a sunny day.
Although the Visitor Center at Glendalough doesn't open until 9:30, the monastic site itself is always open, so if you can time your visit to arrive there early you'll have the place all to yourselves until the first of the tour buses start pulling in around 9:45 or so. Besides the monastic ruins there are several very pretty day walks and hikes in the immediate vicinity if you want to stretch your legs a bit.
You could then backtrack a bit to visit the gardens and the manor house at Powerscourt, and then be on your way in plenty of time for the drive to Kilkenny. It's a little counter-intuitive to do it that way, but it's the early start that's key. Getting to Powerscourt early doesn't gain you anything - you really can't see much until everything opens anyway - but getting to Glendalough early is worth the effort. It's a whole different experience when you have the place all to yourselves in the early morning hours. Only thing to be aware of is that the rest rooms are located inside the Visitor Center ...
A nice stop between the two is the pretty little village of Glencree. There's an old German POW War Cemetery there, located in an evocative little glen next to a pretty stream. An old Mass Rock is located along a short path heading upstream from the graveyard.