Welcome to Flyertalk commuter31, it's good to see you here. I hope you will make good use of this forum.

Ryanair aren't totally fixated on precise measurements, so if you had a small laptop bag which went a cm or two over the dimensions and a larger bag, it's the larger bag that may (or may not) go into the hold free of charge. That's not to say there could be someone out there determined to enforce the rules to the letter but that would be the exception. If challenged I'd say that it's definitely not going into the overhead locker, it will go under the seat in front, and if all else fails then the worst case scenario is the near empty bag goes into the hold and you carry the laptop onboard. Laptops can go into the hold, it's not ideal for various reasons but Ryanair's terms and conditions indicates that an exception for personal electronic devices can go into the hold if it's too big for the cabin. I'm not sure they've cottoned on to the fact that under the Montréal Convention they are then liable for its insurance. To some extent this is all fairly hypothetical. On moderately busy flight Ryanair put out a trolley for larger bags and those who walk past with one small bag are unlikely to the challenged so long as plenty of other people put their bags on this trolley - which they usually do.