You can book and pay for your baggage on the Aer Lingus website.
Click here, enter your Booking Reference number and family name and (you can find your Booking reference on the confirmation we sent to you when you booked).
Once you have logged in you will find a menu on the right hand side of the screen. Select Add Bags.
Online Check-in (optional), opens 28 days before departure, provided you are willing to pay for your choice of seats.