Ryanair don't offer seat selection, but do offer priority boarding (free if you only have hand luggage and check in online). The earlier you get on the plane, the better your chance of a suitable seat.
Aer Lingus offer advance seat selection, either at the end of the booking process, or as a separate transaction afterwards (European flights seem to be handled differently than US flights.). Emergency exit and front row seats cost more. Or you can just checkin online and hope a suitable seat is still available for selection when you do.