I assume there still is a master Seniority List which covers every pilot regardless of base and as a result that would be formed by Date Of Joining.
I worked for them several years ago and after 9/11 that was the system they decided on when they were looking for compulsory redundancies. On that occasion enough of us took voluntary severance. Back then were jobs though, easyJet and Go took lots of guys on.
It would be interesting if Aer Lingus decided to exclusively enforce redundancy at DUB only and leave the 'cheaper' employees alone at LGW and BFS who have only several months worth of service.