Upvote:3
Because the baggage is larger than allowed, Ryanair may refuse to allow you to carry it in-cabin, require it be transported as hold baggage, and charge you. Ryanair is well-known to be picky about all kinds of things — like local printing of boarding passes, luggage size and weight compliance — because collecting additional charges makes money for the airline.
On the other hand, sometimes the rules aren't enforced. Read this SE question, answer, and comments.
All in all...no one can say. If you're risk-averse, obtain new and conforming luggage. If you carry the luggage you already have, you may stroll on board with no difficulty, but be prepared as you may be charged.