In my case the ticket was purchased on avios.com, it is on 125 plate and it can not be managed on line on Aer Lingus' web site. For example, in order to assign a seat I had to contact the Aer Lingus customer service team on Twitter. They were super helpful. I was able to do it on Twitter because we the seats we chose were in the free seat selection area. If we had wanted up front or extra legroom/exit row seats we would have had to do this on the telephone because a payment would be required.