As discussed in other threads, we got COVID and had to drop out of our tour halfway through. I wasn't going to do a trip review for that reason, but I decided that I did have some things worth passing on. Anyone who was able to complete the trip is welcome to add any info about the second half to this thread.
Dublin
The old part of Dublin is relatively compact. While there are hop on / hop off busses, most of the sights are an easy walk from the hotel, unless you have mobility issues. The furthest thing we walked to was the Guiness Storehouse which is 1.3 miles from the hotel.
The Tauck tour of Dublin stops at Dublin Castle and Christ Church Cathedral (Anglican not Catholic). The rest of the tour is on the bus.
Trinity College library (the long hall) and the Book of Kells is no longer included in the tour. It's a block away from the hotel. Book your tickets ahead, purchasing them on site is a bit of a hassle. The book of Kells was a bit disappointing for us. They have a nice exhibit about the book, but the book itself can be underwhelming. They change the pages on display every 8 weeks. When we were there, we saw two of what must be the dullest pages of the book. Just text with a few capitals with minor embellishment. It's luck of the draw as to what you will see. For a library lover like me, the long hall was worth the price of admission. The long hall is scheduled for renovation and is expected to be closed for three years after this season.
We did Rick Steve's audio tours in our spare time (download the free app and content to your phone). Part one is south of the river. We did it as far as the Molly Malone statue. After that it seemed duplicative of our Tauck tour. The part we did was worth it. The second (shorter) part covers O'Connell Street, north of the river. It mostly discusses the Irish War of Independence. Warning: Rick's tours are full of Dad jokes.
We enjoyed the Guiness Storehouse. While there is the "this is how we make our wonderful product" aspect to it, there is also a lot of interesting history. Guiness was a pioneer in industrial scale brewing and advertising and the exhibits were quite interesting. It's best to make reservations for your visit in advance on the website - entry is timed. We ate at the 1837 Bar & Brassiere. It was quite good with upscale bar food. Make reservations well in advance on Open Table. There is another bar (Arthur's) which is first come first served and a self service cafe. The view from the Gravity Bar where your get your free pint is the best in Dublin, but unfortunately the Dublin skyline is not very exciting.
We didn't hit any of the museums, although the new Epic Museum got good reviews. It's a modern museum covering the Irish emigration experience.
Belfast
Belfast is about the Titanic and the "Troubles". The Titanic museum is very well done. Lots of information about the political situation in Norhtern Ireland and the troubles.
Donegal
The Giant's Causeway was a bit of a let down. It looks more impressive in pictures. Donegal town is a small town with a small castle to visit. If you are Vegan or gluten-free consider having lunch at Simple Simon which has a choice of healthy salads and sandwiches. Also, the Food Hall at the Abbey Hotel is a good choice. Cafeteria style service with good sandwiches, salads and hot dishes. Look for the hot pink entrance to the right of the hotel. The Belleek factory tour is well worth the time, even if you aren't into fine china. The craftmanship is amazing, especially their woven basket pieces.
Weather
We lucked out with generally good weather, mostly sunny with highs in the 60s. However, everyone we talked to said that this was not the normal. Expect frequent rain showers and wind along the coast. Our TD said that when the previous tour visited the Giant's Causeway it was raining sideways.
Hotels
The first three hotels, The Westin Dublin, The Merchant in Belfast and the Lough Eske Castle were all great. Spacious rooms by European standards. We spent an extra week at the Lough Eske recovering from Covid and the staff was fantastic - friendly and supportive. I only wish they had a wider dining menu. We were getting pretty tired of the choices by the end. We were disappointed not to have made it to Ashford Castle. Reports from others on the tour were that the rooms were smaller there, but otherwise the hotel was incredible.
The Merchant Hotel has a dramatic dining area (The Great Room). It is the ornate hall of the former bank that is a part of the hotel The guest rooms are more modern with some unique decor in the rooms (although this varies a bit from room to room). I would describe the decor as Art Deco Spanish Bordello modern, with some mildly naughty photos on the wall. Our room had a glass door on the bathroom that looked like this:

Returning from Dublin Airport
Be aware that Dublin, like many cities in Canada, clears you for US customs and immigration at the departure airport. So when you arrive at your destination, it's like arriving on a domestic flight. This allows for shorter connection times if you have to change planes in the US. The Dublin airport has the new facial recognition kiosks for Global Entry. Much faster than the old fingerprint and passport kiosks.