Ryanair announced an effort to encourage travelers to check bags. After November 1st, and for the "basic" fare, you can only bring on board one "small" bag. "Small" is defined as maximum size 35cm (13.8 inches) by 20cm (7.9 inches) by 20cm (7.9 inches). Anything larger will be placed in the hold free of charge. If you pay for priority boarding, then you can still bring on board two bags. The details are at https://www.travelandleisure.com …orts/ryanair-baggage-fees.
Somewhat related, this originally came from an article about not leaving your passport in a bag that might now end up in the hold due to be being oversize. In some instances you would have to retrieve the bag after going through passport control.