Maybe Ryanair is good with their math, but i may be wrong...
The definition of "overbooked":
To take reservations beyond the capacity for accommodation
If Ryanair are booking to anything beyond 189 (which they certainly are) then they are considered to be overselling/overbooking. Even if the statistics show that having to deny carriage is likely to be rare, it is still being practiced!
As I said before, my issue is not with the fact that they are overselling (they'd be mad not too). My gripe, is that they have the gall to stick up an advert which is a blatant lie ( with a clause which says "if the flight is overbooked it's because we're transporting deportees or there is a technical problem with some seats")!