In my opinion (I used to work for Ryanair) they get this quick turn around due to this main reasons:
-Huge pressure on all pilots and cabin crew (my girlfriend works for Ryanair) Whenever you have the smallest delay you need to tell exclatly why it happened, and you better find a good excuse.
-The 737 is the perfect aircraft for it. Very low aircraft that makes possible to load and unload the bags without using too much ground equipment.
-Operating in smaller airports reduce the delays and allows the quick turn around. I was working for them in Frankfurt and they were never in time. Just too busy for them to manage the 25 minutes.
-No cleaning of the aircraft in the turn around. Just the crew picking the packages and bags.
I can say that none of the Ryanair employees that I ever met were happy or proud to work for Ryanair, nobody.