I don't know much about the new Spain trip, but that may another possibility.
One suggestion is leave a day early and rest before the tour starts. I did it in Ireland and wish that I'd done it on other trips. I spent the first night in a downtown hotel in Galway, slept as long as I wanted, had a leisurely breakfast and walked around the town at my own pace, then took a taxi to the hotel where the group was staying. The hotel is not located within walking distance of the city but GCT provided a bus for us.
I didn't take the Belfast pre-trip and wonder if that's where the walk with the columns are; I don't remember anything like that on the main trip.
I don't remember any strenuous walking. Glendalough is slightly hilly but there was no rush getting around. There is an elevator in the center at the Cliffs of Moher and you wouldn't need to walk far uphill or the stairs. The cliffs were fogged when we got there so we all just wandered around the center and had lunch in the cafe.
The hotel in Dublin is a few blocks from the center, but there is a tram right around the corner, which I took on a couple of times when my knee was bothering me. There was a woman using a cane and when I saw her and her husband in the National Museum she said they'd taken a taxi. It wasn't far so I doubt that it costs much.
I took the post trip in Dublin and enjoyed the hop on/off bus and walking through St Stephen's Green. I did not do the walks with post-trip group because I didn't think I could keep up but I saw and did what I wanted to in Dublin and enjoyed it immensely.