BFS is a curious one in being the only EU city I can think of to have two commercial airports*, but no presence of Ryanair at either of them, yet the reasons seem to be as much due to airport politics than anything else.
IIRC, BHD would be a non runner due to night restrictions, whereas BFS simply do not need to lower their charges enough to appeal to FR. I think speculation about a base is often overdone anyway. When was the last time any established airline just went in from scratch and set up shop in totally new city without testing the waters first?
DUB seems to be FR's usual testing ground, but that would be too short a sector. GRO, PSA would seem more obvious choices if FR were to start anything at BFS, or is there some good PSO money / other incentives for operating routes within (the island of) Ireland?
*For the pedants - commercial = operating regular scheduled passenger carrying flights. "City" = IATA designated city (and therefore DUS includes NRN aswell as MGL) and not including islands such as Tenerife (not strictly in the EU), Cyprus or Crete.