Depends what you regard as being undesirable, so that's a very subjective question. I would regard somewhere in Italy or Spain to be undesirable as it is further from home (UK) with longer commuting times and less connecting flights to my home airport. Others would donate their right lung to be in these countries. It really can be regarded as a lottery and is dependant on the needs of Ryanair at that particular moment. Bases are as follows, make your decisions from there, all are a possiblity.
Dublin, Shannon, Cork, Kerry, Belfast City, Prestwick, Edinburgh, Liverpool, East Midlands, Birmingham, Stansted, Luton, Bournmouth, Bristol, Charleroi, Hahn, Niederrhein, Bremen, Skavska, Marseille, Girona, Alicante, Reus, Madrid, Pisa, Bergamo, Champino. I believe both Algerho and Calgarhi in Italy will be added next year.