I'm trying to pack carry on only for my upcoming trip to the UK, even taking into consideration that I will be on 3 different Ryanair flights. The backpack I'd like to bring is 15 by 8 by 21 inches, has no internal frame, and can be cinched down to 6 or 7 inches. The maximum for Ryanair is 15 by 7.5 by 20 inches. I'm wondering about that extra inch on the height. It's a pretty squishy pack and I won't be packing it full. Do you think they'll let it by? I've managed it on easyJet before, but this is my first time flying Ryanair.
Haven't experienced Ryanair recently but a no frame, no wheels pack not stuffed should conform to the sizing services. Czech Air challenged my RS Classic that was fully loaded but I could stuff it into the frame with an easy push to get an OK.
If you can fit a sizing frame you should be able to fit an overhead if all the other carry-on folks don't hog the space. If too many carry-on bags try to squeeze on board, some will get gate checked unless you can jam it under the seat.
Ryanair picks up most of their revenue by charging for everything including oversize bags. They have an incentive to flunk your bag because it means extra euro. It will totally depend on how strict the gate personal are. Good luck.
Ryanair's carry on bag limit is 55 cm x 40 cm x 20 cm which converts to 21.6 inches x 15.7 inches x 7.8 inches.
Make sure you check the carry-on limits of your flight to the UK. That 15" might be a problem is they are being strict.
Ryanair also states that they allow up to 90 bags as carry-on. If you board last, and your flight is full, they could make you check it. Priority boarders don't have to worry about this.