Hand luggage

It'll be my first Ryanair flight.
I've a bag that exceeds for 2/3 cm (57/58 cm) the size allowed for cabin luggage only because of the rollers.
Does the rollers count?
Should I buy another bag?

The rollers/wheels do count... if your bag does not fit in the cage then unfortunately you will need to pay for each journey.

Agree with the above said but most of the time the luggage is not measured. So it is up to you if you are willing to risk it, I think you have an 80% chance of being fine.
Last: if are chosen to measure the luggage, I would put it in the measure basket head first, so that only small section of the suitcase doesn´t fit, not the other way around, good luck