28 day itinerary - feedback please!

In general your proposed itinerary looks pretty good to me.
A few thoughts: If you'll be collecting your rental car around midday you really don't need to go all the way out to the airport to get it. There are a couple of outlets along the South Circular Road that others here have recommended. From one of those it'd be a straight shot out of town heading towards your first stop in Glendalough.
Glendalough itself gets pretty clobbered between 9:30 AM and around 4 PM when the tour buses show up, but if you're there earlier or later you'll have the place all to yourselves.
Bantry is actually well located for use as a base to explore the areas you propose. It's a particularly pretty part of the country and one that's usually overlooked by other visitors so good for you for including it in your plans.
For the ROK I'd suggest basing yourselves somewhere along the stretch between Glenbeigh and Castlemaine, ie as far away from Killarney as you can manage yet still central to the northern half of the Ring - including Valencia Island, Portmagee, Cahersiveen, and the Gap of Dunloe. Traffic can be murder getting in and out of Killarney town when the tour buses are rolling but things mellow out pretty quickly once you clear the last of the traffic circles near the town.
Personal opinion, but I think 3 nights in Dingle is overkill unless you want to devote one of the days to a trip out to the Blaskets.
Time and interest permitting consider driving down the Antrim Coast from Portstewart - it's one of Ireland's great scenic drives on a sunny day.

Hi from Wisconsin,
28 days, how lovely.

Smart plan about staying in Dublin a few days after arrival. I also suggest a few days in Dublin before you depart. Over the decades I have found that staying in a large place at the beginning and at the end is great. When you go back at the end you will be able to see what you learned you missed while you were traveling.

I would strong suggest finding five, maybe six nice locations and doing day trips around that location. Not having to pack, move out, find the new place and up pack every day or every other day is a restful way to travel. I always looked for two things: a bed and breakfast with a historic house or a historic hotel. There used to be a Farm House Association that would get you on a farm in a 'real' house. The second thing is that ideally there would be is a spoke of roads heading out in several directions not far from where you do stay.

Ennis is a great place. A small city with shops and restaurants. A wonderful high street. All in the center of County Clare which is filled with things to see. Clifden is also a great place to use as a hub. It is an old market town. And all of Connemara is there to see.

Macroom is a nice hub for the SW.

wayne iNWI

Wayne makes a good point about returning to the big city late in the trip. But you could also drive straight from Belfast to DUB on 16 June, drop the car, and spend the night at or near the airport for your homeward flight the next morning. That would give you a night to add to Dublin at the start, or somewhere else along the way. Your jetlagged arrival day plus two more full days doesn't seem like enough for Dublin.

Driving down from Belfast to either DUB or Dublin would give you time to stop at Newgrange, one of Europe's best neolithic sites. There's a very good day tour there from central Dublin, but that would be an inefficient use of your limited Dublin time.

Thank you all for your input, I appreciate the tips so much. @Robert, that is great to know there's other car rental outlets on the edge of the city, so we don't have to go all the way back to the airport. Because as you probably guessed, we don't want to go anywhere near the city centre on our first time driving in Ireland, being that we're from the US! Our goal is to time our visit to Glendalough arriving after the tour buses and leaving before! Good to know that Bantry would be a good base, so I'll stick to looking for places there or nearby. If it were just up to me, I'd probably skip ROK altogether, but hey, that's compromise :) just good to know how to avoid the crowds.

A few other thoughts as they occur to me:
The monastic site at Glendalough is open 24 hrs., though the visitor center (where the restrooms are) closes at 5 PM. In May/June it'll still be light until very late in the evening so you'll have plenty of time to explore the place at your leisure without the crowds of midday.
Getting an early start to your days will be a good strategy for avoiding both the traffic and the crowds at any of the popular attractions. The Gap of Dunloe is one of our favorite hikes in Ireland, and if you're there before 8 AM you'll have it to yourselves.
For an off the beaten path excursion consider the drive around the Skellig Ring Road between Portmagee and Ballinskelligs, which features beautiful views of the Skelligs, the Kerry Cliffs, and a pretty stretch of sandy beach at Ballinskelligs. Tour buses don't go out there so it's a very mellow and non-stressful experience.