I'm fairly certain an airport's location in relation to the city it serves has never been a top concern for RYR.
No it hasn't. But they will nearly always choose the lowest cost airport regardless of whether its 6 miles or 60 miles away from the citt but of course most of the time it's the further away airport that best suits them. In Belfast's case, City is the smaller airport and more ideal in Ryanair's mind. It's also the case in Rome and Gothenburg, where Ryanair uses the airport closer to the city since they are the smaller ones. (small/closer Ciampino instead of bigger/further away Fiumicino) and (smaller/closer Gothenburg City instead of bigger/further away Landvetter).