APD etc.
Level playing fields are almost impossible. Firstly there can be government intervention with the likes of APD (and the UK did have Westminister and, I think European, permission to have the Air Route Development grant a few years ago) then airport owners can do special deals to get airlines to start routes - perhaps such incentives can be broadly based (any new route) or narrowly focused - e.g routes to the US.Then comes the PSO subsidies to support routes to remote regions in the EU. Finally general taxation policy - corporation tax, tax on fuel etc. all play a part
The Irish government, and Dublin airport, have made use of most of these and, of course, APD in the Republic is being dropped to try to help reverse the disasterous drop in tourism numbers, not caused by the recession in Ireland but by the worldwide economic downturn.
the simple fact is that Northern Ireland is the only part of the UK to have a land border with another country or, to put it another way, Great Britain is one fo the very few EU countries not to have a land border