How about this? Just might be a little radical for the NI officials who are the guardians of the "rules"!??
Dublin Airport now takes almost a million passengers a year booking in NI/UK jurisdiction because, as much as anything, they are actively exploiting zero tax in RoI v the air tax being charged in NI.
Alongside that there are about 3.5 million passengers departing from the three NI airports (about 7 million airport passengers overall), and there is no tax on the small existing proportion of long haul travellers.
So for the sake of simplicity the APD take from NI bookings is about 3.5 million times £13 - around £45 million per annum.
So, why not simply take control of the air tax in Belfast, and tax-raising powers related to it, and charge a tax of £45 - £50 for all bookings taken in NI for passengers departing from RoI airports, and use the funds raised from this to wipe out the equivalent financial loss to the NI Executive which would be incurred while reducing APD on all passengers departing from the NI airports to zero?
Hey presto, existing and new services from the airports in NI become stronger and more sustainable, more jobs in NI, substantially more spend from tourists and visitors coming into NI and the rising tide lifts the NI economy, simply by turning the table around to a different angle.
If the current loss of business from NI to RoI was occurring the other way round, this would have been a guaranteed move long ages before now.