If you avoid check in by using priority boarding you will get away with the bag size, but you could be taking a risk if you are bringing a laptop bag as well, as the gate staff could assert that you have two pieces of hand luggage and make you return to check in, which adds cost (sometimes more than the ticket) as well as time.
At the check in desk you may be asked to use the sizer if the bag looks big to the operator.
They also tend to weigh hand luggage at the desk so beware the 10Kg limit, they will make you check it if it is overweight.
I have seen people on Ryanair with both laptop bags and hand luggage (sometimes improbably large) so I suppose it's down to the individual employees on the day.