See this is the madness of this, NI would have the best of both worlds they would be part of the common trade area but also a member of the UK there would be absolutely no trade restrictions on them at all. If the DUP weren't so short sighted they would see economically this is a bonanza for the six counties, im sure companies would be tripping over themselves to set up in NI.
That isn't how it would work - under a separate NI system there would be
more trade restrictions on them (the EU wouldn't let the UK use NI as a back-door into the EU, nor would the UK let the EU do the same). They would fall under and be expected to follow both EU and UK rules around investment, government support, trade, employment standards and the rest.
The only thing that would not be affected would be exports in to NI for use there - so basically formerly NI-based manufacturing, service or other industry that did any business outside of NI would be better to base itself in Eire or in the UK because it would be easier to trade with NI (there being no restrictions) and the rest of the UK or EU (where there are no restrictions either on internal trade) if you were based outside NI.