It's not rare that some staff are checking that passengers have all needed documents as they head to departures. Looks like they could check app usage there. They could also easily check the app when bags are checked in (that's Ryanair staff doing this).
In smaller airports (the ones Ryanair used to operate from) it's also sometimes their own barcode scanners before the gates that are dedicated to them.
I believe they will be able to enforce this in many places.
Ryanair use scan-and-drop self-checkin of bags in many airports.
It would also be curious to see who pays for removal of persons once they are airside - eg in the case the flyer with nothing to check in who goes past airport security, but before RyanAir staff meet them at the gate.
In smaller airports (the ones Ryanair used to operate from) it's also sometimes their own barcode scanners before the gates that are dedicated to them.
I believe they will be able to enforce this in many places.