score:4
No need to worry, you'll be fine.
First of all, the baggage sizer is slightly larger than the allowed dimensions (by a couple of centimeters in each direction), so even if someone decides to check your bag, it's going to pass.
That said, Ryanair used to be very strict about baggage, but that was > 5 years ago. Nowadays, they're no stricter than the average airline. I have never experienced a systematic size check of every bag on the plane. Typically, they only send people with very visibly oversize luggage to the sizer, but sometimes not even those.
(However, note that what I said above baggage size doesn't apply at all when it comes to tje item count. So don't try to smuggle too many pieces of baggage on board, that's likely to end up in a costly failure.)
Finally, most of this is even less of a concern for you as it used to be. Because you have paid for priority (unlike many other travellers), the airline doesn't want to discourage you from doing that again next time. You can thus count on even less scrutiny than you'd normally expect on similar airlines (or on Ryanair before they changed the baggage policy).