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Formally it's the Stobart rules that apply.
Aer Lingus aren't mindless at NCL, so if it's a bit over then you'll get it on board. The CWS Skypak HBO bag, which is just under the BA limits, fits snugly in the ATR overheads by dint of its lack of rigidity, and under the seat, less comfortably. They do collect larger wheelies at the front of the aircraft and return after the flight at the apron in DUB.