The baggage size and weight are set by the operating carrier

If you are flying on Aer Lingus regional (4 digit flight number starting with 3) then its an ATR-72 with restricted carry on sizes, no gate check is offered, 48 x 33 x 20cm

The regular Aer Lingus carry on size is bigger than AA at 55 x 40 x 24 cm vs 56 x 36 x 23 cm

The personal item is always 25 x 33 x 20 cm

There is some flexibility in width but it has to fit under the seat and if its the ATR the window seat has little if any underseat space available, whereas the isle seat has lots of space

If its a A320 flight, 3 digit flight number you can get a way with closer to 40cm in the width under seat

Personal items are not allowed under the seat at any exit row