I believe it cost Aer Lingus around £100,000 to put each 330 on the British register.
At Belfast, they need only 1 320 for the current schedule.
With the additional cost of each pilot go through the CAA layers of bureaucracy to obtain a UK Licience and an Operators Conversion Course which means that they cannot fly an EI registered aircraft so new pilots are required when the aircraft is replaced for maintenance.
I’m sure that Aer Lingus have decided that the aircraft can make more money elsewhere.
its another own goal for Brexit where the only winners are the Caa’s coffers, not the consumers.