I think an AGP base would run into union problems if it was operating services to Ireland. In the same way that LGW-based crews can't operate into Cork and Dublin, they would also be restricted. I guess flights into Belfast and Knock would be OK though.
I've always been a bit surprised that Ryanair and Easyjet haven't had more presence in places like Malaga and Faro. Ryanair have a base in Alicante, but
AFAIK that's the only base either airline has in a traditional holiday area. Granted, you'd probably have to operate it with fewer aircraft in the Winter, with knock-on affects for staffing, but you could possibly get round that by having aircraft from those bases operating year round and bumping up capacity using aircraft from other bases in the summer.