I don't think there's a single definition.
You could either define it in terms of flight times or the crewing situation i.e. Can a crew operate a return flight (Short-haul), an entire leg (Medium-haul) or are two crews required (Long-haul)?
I would assume Egypt from Cork counts as Medium-haul.
Regarding the EI A321, hopefully this isn't a sign of bad news for their Gatwick presence. It's not in the system yet, but then neither are Munich, Alicante or Nice (and whatever about Munich, I can't see the last two being dropped). Other parts of next summer's timetable on the Aer Lingus website suggest that Rome is back and Amsterdam maintains its Winter frequency through the Summer.