I think it depends on who you get on the day to be honest. In my one and hopefully last experience with ryan-air i saw several people in the boarding que get their items tested and rejcted but also saw many who shouldn'tve gotten through get onboard. This was in Madrid airport and there was really no consistency.

How large it is might determine if you get it on or not, if it's only a few cm longer i doubt it should be a problem, but you never know!