If you join as a direct entry then you'll stand way better chances to have the preferred base than people that joined the company even years before you, especially if you are a FO.
Plenty of angry pilots all around the network that have been flying X years for Ryanair and never had the chance to be based where they wanted.
They just implemented a "live chat" for the basing, which is completely useless, as they won't tell you anything, they can't discolose what position you are in the "list" (does that even exist?) nor give an estimate for the base transfer. However at the same time of you asking info about your dream-base there is probably a DEC/DEFO (or even more than one) that is signing a contract (he/she even got to choose what contract!) for your requested base.
Biggest maintenance bases are STN DUB BGY PIK and MAD. Probably new maintenance bases coming in the near future.
Learjets are based at STN BGY and MAD.