Very interesting
"Unlike other airlines, Ryanair does not overbook flights," a spokesman told The Irish Times. When he was referring to other airlines, he might have had Aer Lingus in mind because it does allow for overbooking although a spokeswoman assured us that it was "rare".
“As with all other airlines our flights can be overbooked from time to time,” she said. “This can result from operational disruption where a lower capacity aircraft is substituted for the one originally planned or sometimes, where a particular route has a high level of no-shows, bookings may exceed the number of seats available.”