There used to be something called a (wait for it....) "Flight Uncheck Fee" for unchecking in from a flight! I can't see any mention of this on the website anymore.

In the worst scenario, you can always pay for checked luggage at the airport, albeit at a higher fee.

Remember Ryanair now allows two carry on bags (although the second is quite small) and they are far more relaxed about the maximum size than previously. Nowadays, the measuring "cage" gets wheeled out before some flights but they rarely ask anyone to actually put their luggage into it.