I've traveled all over Europe and Canada, both on bus tours and on my own, and personally, I prefer to follow my own schedule. HOWEVER, if you've never been to Ireland before, I would recommend that you and your spouse take the bus tour. While it is great to be able to do your own thing, you will definitely see so much more if you have someone guiding you, and it's also really nice to have your guide explain the history behind the sights. I think most (if not all) bus tours give you time to explore on your own as well, so it's kind of like the best of both worlds! I've also been lucky enough to not be on a tour with inconsiderate people who keep the group waiting, so hopefully you'll be lucky in that aspect as well!
I'll admit though, that the schedule can be quite hectic, especially if the agenda states that you'll be seeing lots of places in a short amount of time. I remember being exhausted at the end of the day, only to have to wake up early the next morning to travel to the next city/country. If you enjoy a more relaxed pace of sightseeing, bus touring probably isn't for you
As far as the age issue, let me assure you that you will NOT be the youngest people there. Obviously not the oldest, either, but you certainly won't feel out of place. I'm 23, and I've done bus tours in the past with my parents (who are in their 60s), and there were always a handful of people who were in their 30s and 40s, and they sometimes brought their older kids (around college-aged). Certainly not the "let's hit the bars" kind of folks, but they were all very pleasant... And if all else fails, just ditch 'em the first chance you get and meet back at the bus at the specified time
So my advice to you would be this: If you've never been to Ireland before and you really want to see all the sights, take the bus tour. I used to complain to my mom every time she would drag me along on one of those tours, and she would always say that tours (of any kind) are the best way to get an overview of a place you've never been to before. In retrospect, I realize that she's right. It's like reading an outline of a REALLY big book, so that the next time you visit, you'll know what you want to investigate further
Regardless of what you decide upon, have a great time in Ireland! I wish I was going, too!