You need to add about 25% to ViaMichelin's time estimates to come up with a more realistic picture.
There are three routes from Dublin Airport to Glendalough. The times range from just over 90 minutes to near two hours, depending on which route you choose.
Glendalough to Rock of Cashel will be closer to three hours.
Rock of Cashel to Kenmare will be closer to 3h 45m.
You aren't going to get out of Dublin Airport much before 9:30, and you could still encounter some rush hour traffic on the ring road. You will want time to actual see what you came to see, not just drive by, so allow at least an hour at Glendalough and maybe two at Cashel for that. You will want lunch at some point. WITHOUT stops, you are already looking at getting into Kenmare around 6:30. With an hour at Glendalough and two at Rock of Cashel, you're now at 9:30 and haven't allowed any time for lunch or other stops.
You're the only ones who can say whether it's doable or not. Looks like about nine hours of driving plus sightseeing plus at least one meal. If you want to do it, I would suggest having lodging arranged in Kenmare, and making sure that they know you will be arriving late. What time of the year are you planning this?