Day 2 -- this seems like too much, too far spread out. I haven't been to Glendalough, but I believe you can spend quite a bit of time there. My husband and I did both Cahir Castle and Rock of Cashel in one day and it was a nicely paced day, with an overnight in Kilkenny. You might be able to see Cahir, Rock of Cashel, and Kilkenny Castle in one day coming out of Dublin (even that is probably pushing it because sites don't stay open very late in Ireland). I'm doubtful that you can visit Glendalough and enjoy it, while trying to fit in the rest of your stops. Via Michelin gives me a road time estimate of 4-1/2 hours for the stops you listed. Factor in time to pick up your rental car and a quick bite for lunch, and you'll be racing the clock and not enjoying yourselves. We stayed at Fanad House in Kilkenny. Nothing extraordinary about the place, but it was nice, cheap, and clean with a good breakfast.
Day 3 -- Bunratty Castle and Blarney Castle are on opposite sides of Killarney. Again, it seems like an awful lot of driving and not much time for enjoying. I would pick one or the other and spend the rest of your time in Killarney. You may want to check out Muckros House and Ross Castle in Killarney. I haven't been to Bunratty, so I really can't compare but I can tell you that we thought Muckros and Ross Castle were fantastic. From what I remember of Blarney from my college trip, is was a very touristy "check this off my list" kind of place. I remember the grounds/gardens being beautiful, but I was on a bus tour and I didn't really have time to explore. As far as lodging goes, check out Friar's Glen in Killarney. We absolutely adored this place. Best B&B of our entire trip. Luckily for us, all of them were good, but this one was outstanding.
Day 4 -- For what it's worth, my husband and I spent a complete day driving Dingle Peninsula and still didn't have enough time to do everything we wanted to do. The sneaky thing about Ireland is that around every bend in the road there is something new and wonderful to discover and enjoy. It's a two-sided thing...one side, you'll get to enjoy it no matter what. The other side, with this time-frame, you just don't get as thorough of as an experience that it deserves.
If I were you (with your current prioritized list of sights), I would do this:
Day 1: Dublin
Day 2: Killarney - visit Rock of Cashel and either Cahir Castle or Blarney Castle on your way to Killarney.
Day 3: Dingle - spend day driving and exploring Dingle peninsula.
Day 4: Dublin - drive back to Dublin, see Bunratty Castle on the way. You may have time for another stop along the way if you find something that catches your eye. Via Michelin says it's about 5 hours of drive time between Dingle and Dublin.
We loved Kilkenny but I just don't think you have time for it. I think what I've suggested will allow you to see several of the things you want to see in a logical (map-order) way while giving you enough time to enjoy some things. Via Michelin will have more accurate road times than Google, FYI. Hope this helps!