Although I've wondered how much research actually goes into Ryanair's route planning, I think its entry into the Spanish domestic market is interesting and may work. Fares on routes such as Madrid-Alicante and Madrid-Valencia have historically been high but, with the Spanish economy in trouble and fast rail links being constructed, Ryanair's move may have come too late. Iberia has responded by offering breathtakingly low (for them) fares on selected flights, presumably to hit Ryanair. (Personally, I'd choose Ryanair over Iberia any day, although the fact that the latter uses the gorgeous Terminal 4 at Barajas is tempting!) The truly huge market is, of course, Madrid-Barcelona, but air travel on this route seems to have been impacted by high speed rail (leading Spanair to link with Renfe and offer joint air-rail tickets). Nevertheless, these trains are expensive so if Ryanair could find some way to fly to el Prat and give up on pretending that Girona and Reus are "Barcelona" airports......
P.S. As I've written before, Ryanair should look at Salamanca. There're a number of different markets (such as students and tourists) and the fares charged currently to fly to/from the city are astronomical.