Basing is a complete lottery, although if you want STN or DUB you might have a very slightly better chance of getting it due to the higher turnover of crews. Generally most people get offered a choice of 3-4, when I was based I got to pick from STN, PIK, SNN, HHN & MRS.
Once your there though moving bases is a bit hit and miss, i'm on a transfer list at the moment for a base and am getting pretty narked that new cadets are going there ahead of me! Sit and do your time and the opportunity comes, but with everything Ryanair be prepared for anything.
Horgy