In fairness to Aer Lingus Regional it's unlikely that all that many airlines would be in a position to magic up the two buses that would be needed to transport the full contents of an ATR at 11pm or so. Relying on existing services probably makes the best sense, with the understanding that the airline is responsible for costs that customers incur.
Apropos of nothing, I'd personally suggest that a taxi split four ways is a reasonable expense at that time of night if the alternative is a bus at 2:30am.