I spent 24 days in Ireland in June, and I would have enjoyed spending an additional night in any of the places you have on your itinerary. I agree with your decision to skip Waterford as a place to spend the night. I regretted my night there and wish I had spent another night in Kilkenny instead and toured Cashel from there instead of from Waterford.
You didn’t state what your main interests are, so I’ll offer a few ideas based on possible interests.
I spent 2 nights in an Airbnb outside Portmagee (ROK peninsula) so I could have more time to view the ROK and take the tour to Skillig Michael (one of the highlights of the trip). It was a 15 minute drive to the pier where the boat left. These need to be booked in advance or show up early to be at the front of the line to get the last minute cancellations spots. Also, nearby Valentia Island is fun to tour, with great ocean cliff views that are less crowded than some spots. You could add an extra night near Portmagee or in Kenmare if these options interest you.
I assume you’ll visit the Cliffs of Mohr on your way from Dingle to Galway. I spent 3 nights in Galway so I could spend the day exploring the cliffs and arrive in the evening in Galway. I enjoyed taking time to walk along the cliff trails farther than most of the tours did. With more time I would have toured The Burren more; it’s a very different landscape.
Also, on one of my Galway days, I took a tour to the Aran Island of Inishmore which takes up most of the day. It was well worth it; my guide was Gabriel Faherty whose van is labeled “Boats to Goats.” A former fisherman, he now makes goat cheese on the island and had just won an award in a food competition. He was very entertaining and knowledgeable.
So depending on your interests, you could add another night in Galway to explore more of the countryside or an island; otherwise 2 nights is enough.
From Westport, I went to Sligo because I am a big W.B Yeats fan and wanted to see his grave at Drumcliffe Church and the awe inspiring view of Ben Bulben mountain. Sligo also has a nice river walk with good restaurants and coffee shops with river views. It has a nice abbey to visit as well. One night and part of a day would be enough.
Several people have recommended Trim (near Dublin), and I agree it’s worth a night. From there, it is a short distance to Bru na Boinne (the Newgrange mound). Going inside the mound was fascinating, and Trim has a wonderful river walk with scenic ruins and a grand castle.
As to pre-booking, even in June, I saw B&B's with vacancy signs as I drove into the towns on your list. Some were a few miles outside of town, but they looked nice. For Dublin, Galway, and Dingle, you may want to pre-book.