A lot of people will probably think your itinerary is a bit rushed, but I made a similar trip like this in May a couple of years ago--in the opposite direction, going counterclockwise around the island. We did not visit Waterford, Kenmare, Galway, or Sligo. Our itinerary included, in addition to the places you listed, Kinsale, Donegal, Doolin, and Killarney. (I would have chosen Kenmare over Kilarney, but my son had to catch an early morning train to Dublin the following day, so ....). --Anyway, we had several "one nighters" as you do, and though I would have liked to have had more time in every place we visited, ... it just wasn't possible, and I don't think it was too too much.
My recommendations, of course, are based on my own limited experience. This was our first trip to Ireland, so the recommendations of those with more experience will be more valuable, but here goes:
Dublin: If you have time, drop into the Little Museum of Dublin and get a guided tour. The museum is (or was) tiny and will not take up much more than an hour. (It's on the Hop-On, Hop-Off bus route). The museum has a significant focus on the more recent history of the city, something, ironically, I discovered I knew much less about.
Trim: The walk described in the RS guidebook was wonderful (It picks up near the castle and goes along the river.) We took it on a lovely afternoon after we arrived at our B&B. We had spent our first three days in Dublin, and this countryside walk was such a lovely contrast. (We enjoyed Dublin too, but it was really nice to get out into the country and see a completely different Irish landscape and locale.) We had a memorable evening in the James Griffin pub (also mentioned in the RS book or it was). Met friendly locals (some of who joined us at our table with our newly met solo traveler/now friend from Australia), heard great music, and got to participate a bit in some singalongs. Our experience here was what I had hoped to experience in Ireland just once. As it turns out, we met so many memorable people on our short journey.
Belfast: If you are interested in "the Troubles," consider getting a private tour. We did, and I am so glad we made that decision. We spent an afternoon learning so much.
Derry: Read up on parking. We never could find a place to park and had to miss the city walls. (Derry was not an overnight stay for us.)
Kinsale: I loved this town. Everything in the RS guidebook was accurate. We did the historic stroll and the ghost walk described in the 2017 guidebook, walked to the fort, etc. The ghost walk was fun (comedic). We enjoyed it. It was an entertaining way to spend some time after dinner. The historic walk was even better--serious, well-paced, and led by a knowledgeable, enthusiastic guide. We also really enjoyed our Airbnb, which was in an old temperance hall right above the main street. Great hostess, who told us how to get to the true "Old Head of Kinsale." The views were beautiful, and if you are familiar with the Lusitania's sinking even more meaningful.
Kilkenny: I think the tour we were granted in the Medieval Mile Museum (by a volunteer who had time to spare) may have been the best we have experienced in any place.
Drive through the Black Valley: A couple from Dublin whom we met at a pub in Dingle (also a great town as you know) told us that it was possible to drive through the Black Valley. We did. It was harrowing and thrilling, but we succeeded in finding a church described by the couple. (They had told us an interesting story about the wife's grandmother and her connection to the U.S.) The drive through the Black Valley was the most beautiful of all the scenic drives we made, but my husband, who was driving, enjoyed it far less. I'm not sure if visitors are encouraged to make the drive, but our Dingle friends assured it that it could be done, ... and we did it.)
