Irish Citizens are an unusual case in terms of right to live and work in the UK - it is unrestricted and independent of EU freedom of movement. UK citizens also have the right to work, travel and live in RoI, again independent of EU treaties. So EI-UK, or any other employer, is perfectly entitled to recruit Irish citizens to work in the UK.
At the moment EI seems to be redeploying pilots based in DUB/ORK/SNN to work for EI-UK at MAN. It works both ways, I don't think there were any compulsory crew redundancies when the LGW base closed.