I suppose a lot depends on how much flying you guys want to do, and how much time you have.
Donegal is nice, especially the coast, and Donegal airport is very GA friendly, but it is in the middle of nowhere. There is nothing within easy reach apart from the beach.
Letterkenny (Co. Donegal) is on the edge of Letterkenny town, and is very GA friendly. However the runway is very narrow (Just about the width of the undercarriage of the PA28-200R that I fly
). If you're feeling confident, then it's a good spot to stop at (but no AVGAS).
Sligo is GA friendly too, has avgas, and is within walking distance of Strandhill village. There is a lovely beach there (popular with surfers) and a few places to have lunch.
Some of the most beautiful scenery that you'll see from there air, is along the coast here as shown below.
Flying over all this little islands in this area is fantastic.
This would bring you the Arran Islands which Thing mentioned above. They are a wonderful place to stay overnight, though don't expect 5* accommodation. A B&B/Guest House is pretty much all that is on offer. The runways are about 500mtrs (with a little overrun too) and are hard, but have a few loose chippings, so be careful where you do your power checks. There is a runway on each of the three islands, but InishMor is where you'd want to stay overnight, as it is the biggest of the three islands.
Cycling around them is popular, but be aware that they are hilly, so it's hard work!
Others have mentioned the Islands off the west coast of Scotland. I was going to say them too, until you said you wanted to come to Ireland
dp