I doubt very much that IALPA will let this happen but then EI could always set up another business that would make it a different entity and outside of IALPA's influence.
If they aren't on the Aer Lingus seniority list they aren't Aer Lingus pilots. If Aer Lingus wants to set up another airline - fine. Just as long as they don't try calling it 'Aer Lingus'.
They've had subsidiary airlines before - e.g. Futura. Wholly owned by Aer Lingus, employed non-Aer Lingus pilots, no problem.
But would Aer Lingus management seriously want to devalue the Brand by dropping the name? What would the share-holders think!