I'd imagine that Ryanair came to the decision to purchase new planes on its own. Why? I suspect Ryanair's plan was to obtain used 737-300 and -400 aircraft which were being parked/returned to lessors. I don't believe they had plans to buy 737 classics (i.e. -200 series aircraft) due to higher maintenance costs and lower fuel efficiency.
The problem, of course, is that while about 250 737-200's have been parked in the wake of 9/11, there are only about 30 newer-generation 737's (-300,-400,-500) parked; LH won't sell theirs to Ryanair and CO is likely to return theirs to service as the economy improves. So, while they could pick up a few on the used market, they likely couldn't find enough to expand the fleet as extensively as they'd planned. Moreover, since the newer 737's are still in relatively short supply, I'd imagine that the leasing/purchase costs haven't fallen very much.
I think Ryanair made their order a competition between Airbus and Boeing in order to obtain the best price possible. Simple as that. If Airbus had made them an offer which Boeing was unwilling to match, they likely would have gone with Airbus.