AOCMALLOW,
NI politicians and business people are entitled to lobby for any economic change that brings benefit or advantage to NI plc. As already highlighted in this thread, UK Customs & Excise already exempt some Scottish airports from APD, so the precedent is there, and the EU have no power to counter it. Further, if APD is devolved to Scottish, Welsh or NI legislatures, then those bodies could apply it as they see fit.
You seem quite annoyed that "people in NI pick and chose (sic) the parts of the UK they like". I take it you mean the parts of UK legislation rather than the towns / cities / regions? Well, anyone will complain about some facets of life in their own country. It doesn't mean you don't lobby to change it.
ROI will rightly fight it's own corner...so don't try to deny NI citizens the right to call for changes that will work in our own favour either. In the context of the UK, NI has a reasonable argument for reduced APD. We don't need to justify that to Dublin...it is our governments responsibility to ensure all parts of the UK are competitive, and if lower APD in NI helps to achieve that, then we are entitled to argue for it.