I believe there are very few situations in which they do, certainly an extreme minority in terms of total flights.
I'm afraid you believe wrong!
I agree that there are few occasions when it is necessary to deny boarding, but the majority of Ryanair's flights have a sale lid that is set in excess of the aircraft's capacity! If a flight isn't overbooked, it is purely due to the fact that these oversale fares have not been purchased. As somebody else has mentioned, Ryanair's yields department are not stupid and will endeavour to make good use of their high no-show rate.
Again, I'm not criticising Ryanair for overbooking, they just need to stop using this underhanded method of advertising!