EI is a unionised airline just the same as BA. Therefore it has a host of long fought agreements. One key agreement covers seniority. It matters not a jot if a pilot on the seniority is British, Irish or any other nationality when filling up the MAN base. There is a bid form and seniority. The most senior person will get the gig. The only caveat is the right to work in the UK that I understand will have precluded some following Brexit.
People that struggle with the concept of seniority should stay away from legacy airlines.