Having taken five Rick Steves tours that were all terrific (but not to Ireland), I am obviously a fan and would probably choose one of his tours. His tours are my first choice inside Europe.
I have traveled once with Gate 1 - a wonderful tour of southern India. I would certainly travel with them again, particularly outside of Europe. One caveat - This was one of their “Discovery” or “small group” tours, limited to 22 travelers. Their standard tours are cheaper, but can have as many as 40 travelers - too many for my taste.
I have also traveled twice with OAT (Overseas Adventure Travel) to Myanmar/Laos and Morocco, and these were also very good tours. They limit their groups to 16, so they are probably more expensive, but not crazy expensive.
I have heard good things from other travelers about Odyssey Unlimited and Road Scholar.
When choosing a tour, you need to look at the details carefully and focus on what is most important to you. What is the itinerary? How many nights do you spend at each location? Where is the hotel located? How much free time is provided? Does the tour cover your “must-sees” or will there be free time for you to do these on your own? What meals are included? Are there optional tours that add to the cost? What is the tipping policy and how much will that add to the cost? How many people will be in the group? What is their cancellation policy and what was their track record during the pandemic? You get the idea.
Also, I always add some independent time before and/or after the tour. Never regretted doing that!