The Aer Lingus cabin baggage limit for short haul flights is 44 liters, 10 kg, longest dimension 22 inches. For regional flights it is 36 liters, 7 kg, longest dimension 19 inches. I'm not sure which category your itinerary falls into. In any event, even if you have a 44 liter pack, you are not likely to have it filled to bursting, so that will be less.
Also remember, you can take a "purse" in addition to the bag. One of the small collapsible (into a package the size of an egg) packs works well for this, and any surplus can go in that.