In most cases, airlines don’t officially allow cabin swaps between tickets in different classes, especially long haul. Business class comes with different service levels, catering counts, and sometimes security or manifest considerations.
That said, sometimes crews will allow an informal seat swap once onboard, but it’s entirely at their discretion. You can’t just switch boarding passes before departure unless the airline reissues the tickets, which usually isn’t simple.
Best approach:
Call Aer Lingus and ask if they can officially move your friend into your business seat and downgrade you to economy on the booking.
If that’s not possible, speak to the gate agent before boarding and explain what you’re trying to do.
If needed, ask the cabin crew once onboard, but be prepared for them to say no.
If they do allow it onboard, expect that all business class service would go to whoever is sitting there, so you’d effectively be taking the economy seat and service.
It’s a really thoughtful gesture. Just don’t assume it’s automatically allowed without checking.
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