Never been charged, have been on flight with crutches. They usually get you to put them in overhead bin so not in way of aisle etc. Can't say if Ryanair charge...but if they don't already I'm sure they soon will think of a way to.
Regarding security I had to put a walking stick through the X-ray scanner going on a plane a few months back and hobble through the security frame but was on anothr flight was allowed walk through the frame with crutches. I can't help but think of Freddie Forsyths "The Day of the Jackal".