It's not that their AOC is expiring ( their UK one is valid ), it's that the CAA are finally saying that they can't continue domestic UK ops on their Irish AOC. They've had nearly two years grace in order to sort it out.
Their UK AOC is held by Aer Lingus (NI) Ltd who ironically enough are headquartered at Belfast City Airport, though they only have the A321 and A330 on the cert which are operated from Manchester. It is difficult to see the logic, Captain...
Meanwhile Emerald Airlines UK Ltd, headquartered in Newry, are still waiting for their UK AOC to be issued. AT76 G-CMJN has done route-proving and is parked at EGAC awaiting the paperwork for revenue services.