Taken from Aer Lingus web site:
Standard Allowance
One single piece of cabin baggage.
All carry-on must be within the size dimensions of 55cm x 40cm x 24cm.
Aer Lingus Regional (flight numbers 3000 to 4000 operated by Aer Arann) size limits are 48cm x 33cm x 20cm.
FlyingOnceMore - where did you get the dimensions (48cm x 36cm x 21cm) for their regional flights from? Is Lingus publishing two different sizes for regional flights?