Ryanair. If you don't pay for a seat, you may have to queue to get a boarding pass

[GAAmam]

Current detail from their website (FAQ):
DENIED BOARDING
Ryanair, as a policy, does not overbook its flights. However, in
the unlikely event that a seat is not available for a passenger
with a confirmed reservation, we will seek volunteers to
surrender their seats in exchange for benefits that we and the
volunteer may agree upon before involuntarily denying
boarding to other passengers. If there are insufficient
volunteers and we deny you boarding involuntarily, you are
entitled to the rights set out below.

 

[michaelm]

I expect that queuing up to have a seat assigned also gives the staff a good opportunity to object to your carry-on and charge you for same.