2 seniors want to see Ireland, Scotland and England...2+weeks. Can you recommend a reliable agency for this?

What's your budget? Private driver-guides will be expensive (but worthwhile if you can afford it), and a company such as Celebrated Experiences can put that together for you.

2 (or even '2+' ) is not enough time for three full countries - you really need to focus a bit more on what you want to see/where you want to go.

There are - quite literally - thousands of places worth visiting and you could easily spend your two weeks in just London and a bit of southern England, or just in Scotland, or just in Ireland. So before you start hunting for group tours and agencies -- think more about which part(s) of these countries you want to visit. THEN we can give advice for the best way to do it, which are the best tours, etc.

>>Booking into small group tours in all three destinations likely won't be possible.<<

Of course that is possible -- just maybe not advisable to try to squeeze in too many too far apart. Rabbies for instance does small group tours in all three countries from one day to 8 or more days. But except for Scotland where they go just about everywhere on one tour or another, their offerings in some other areas are more limited.