If you've checked in online for the Aer Lingus flight, do the following
- Follow signs for the baggage hall
- When you reach the baggage hall, follow signs for connecting flights (up a flight of stairs at one end of the baggage hall). Do not proceed into the arrivals area
- At the top of the stairs, you'll have to pass through security again, but you'll be at the front of the queue
- Proceed to the gate
The only exception is if both flights arrive and depart from pier A, in which case you'll be able to walk from gate to gate.
If you haven't checked in online, you'll have to go to the check-in area.
You might want to be aware that pier D is quite a trek from the baggage hall.