Well that's not true now is it?
It's considerably more than a "couple of Euros", it's set from London for the UK and NI with some dispensation to UA's BFS operation as it was killing it, and Scotland's Route Development Fund was wound up before it was ruled illegal state subsidy.
Think EI means the difference now, 12 pound to 3 euro which is around 9 pound or so.
We all know it was set form London but Berlin, Brussels, Rome also have taxes like this one and all have borders, but as said above the ROI had it before the North and it didn't make carriers flock to Belfast from DUB, nothing has changed.
Yet again people still don't understand the small population of IRL means only one airport will be a hub. Belfast has hardly no other routes besides sun and UK ones and people expect Long Haul carriers to operate lots of flights. Come on people, it will never happen.
In the UK London, Manchester will be the main LH hubs while GLA/EDI will be to an extend. Other won't but they will have there NY flight.