Erin,
How "low-ish" are they mobility wise? I'm guessing you reference that, just to indicate that an active Rick Steves tour would not be a good fit.
But, would they be huffing/puffing after or have a hard time walking a slow half mile? Would they have a hard time climbing into a tour bus?
On a scale of 1-10, with 10 being someone who could run a mile (and keep going afterward), and then 1 being someone who would need help climbing into a tour bus and would be winded after walking 50 feet, where would you place them on the mobility meter?
If they would be 5 or 6, then a Tauck Tour might be perfect for them. They would likely enjoy the fellow tour-goers, and Tauck (unlike the Rick Steves' tours, handle all luggage for their guests, would stay in places where it would not require someone climbing stairs to get to their overnight room, and they stay in lovely hotels, have great meals, etc. They might particularly enjoy one of the Tauck Tours that stay at Ashford Castle (the previous home of the Guinness family).
Once someone goes on a Tauck Tour, there is a very high likelihood they would become repeat customers. Tauck's greatest promotion is the wonderful word-of-mouth from happy past customers. You can check out their Ireland tours at www.tauck.com
But, if your parents might not be up for being on a group tour for multiple days, then I do know a wonderful private driver (who we used for a last-minute trip to Ireland a few years ago. Tony McGann of Doolin Taxi is wonderful. Very personable, great sense of humor, reliable, on-time, safe, and he also served as a bit of a tour guide for us. I had a basic itinerary in mind, but I also told him to just show us the beauty of his country, and he sure did deliver!!! We recommended him to friends, and they absolutely adored him, too.