Agree with the previous poster. The gate staff is too busy looking for full-sized carryons among non-priority passengers (in order to charge them extra), they will unlikely care about 4 extra cm.

Since you are flying them for the first time, don't forget that with Ryanair, you have to pay extra ("priority and two bags") to take a full-sized carryon aboard, such as yours. Otherwise, you can only take a really small bag into the cabin (40x20x25cm, about 1/3 the size of a regular carryon).