The annoying thing about AerLingus is that it's not as clear as on the RyanAir site when you begin.
They also MAKE you pay to select your seats on flights. I recently booked flights and paid for the flights as one part of the transaction. It advised from the outset that you 'could' select your seats when you had completed the transaction. However then I was presented with the screen to select seats, it wouldn't let me proceed without selecting and so I had to pay an extra €12 just for choosing seats!?
You can book a flight and not pay for seats. Seats are then allocated at check in...You dont HAVE to pay for the seats.