Only itineraries booked on aerlingus.com can manage things like seat selection on their website. Otherwise you need to call a contact center and request seats. Bear in mind that as many people have found, the ticket class you are booked in may only have limited entitlement (or none) to pre-booked seats so there may be a fee.

If it's really important it might be worth doing. Even though you can check-in at T-24 partner bookings frequently can't select seats then either, and you can only proceed through online check-in by taking the automatically assigned seat.