They save on admin staff costs by getting the pilots to be their own secretary and book their own everything basically.
This works on a very limited basis but now the demand to send Ryanair full time contracted pilots out of base has become so great the admin departments that do exist are still (I guess) at the staffing levels when the airline was a fraction of the size it is now.
When an airline grows everything has to grow with it and thats doesn't seem to have happened when it comes to the basic admin areas of the airline.