Thank you everyone for your help and advice.

I guess I am overthinking it, and that rather than worrying about the actual physical dimensions of my backpack I should be thinking about would it fit into a baggage sizer which was 33 x 20 x 25 cm. Both this Cabin Max bag and my other small backpack (which is 41 x 15 x 28cm), will easily fit so long as I don't overfill.

Thanks again, you have saved me spending money on another bag specifically for Aer Lingus flights.