On the three occasions I have flown Ryanair, twice out of STN and once out of DUB, I have always travelled with a backpack closer to 20kg than 10kg, and far outside their stated dimensions. I have never been charged or questioned about it. I have presumably been lucky, but their staff are all subcontracted, and probably not paid enough to necessarily care about squeezing every last bit of money out of the passenger. On the most recent Ryanair flight I took, STN-SXF, at least a third of the passengers appeared to have bags larger than the regulation size, and all of them boarded with no trouble. Your mileage may vary, however, and Ryanair's terms are clear that they have every right to gouge and massively inconvenience you if they decide to notice your oversized hand baggage.