As long it's only one piece ... Although I thought Cryan did have a maximum weight limit.
The one piece limitation is only in effect for that small zone 5m either side of the check-in person. As soon as you pass them you can take out your chockie bar, or your hand bag, or your tuba.
This policy, which other LCC apply as well, does have some on-board consequences. Next time you fly just watch the number of (mostly women) passengers who stand-up to get to their carry-on to get to their hand-bag (purse) so they can buy some expensive stuff. I wonder if they lose sales become some pax just can't be bothered to get to their money/card?