I’d echo the above. My experience with Mainline aer Lingus is that if you have one bag and it looks small enough to fit under the seat, they don’t fuss much.

my laptop bag is bigger than their limits and I’ve never seen the slightest interest shown.

Your only risk might be if it *looks* like it could be too big for under a seat. As you note, the official limits are really small, so if anyone gets suspicious that it wouldn’t fit under the seat and decides to check you won’t have a leg to stand on as it’s clearly larger than the limit.

for what it’s worth, I’d risk it any day. A carry on bag that meets their limits would be too small to be much use for a multi day trip, so not worth the effort to find one imho. Unlikely worst case scenario is that they charge you at the gate to put it in the hold and then lesson learned and you’ll check it next time.