I am not an expert, but just for interests sake, this is the current situation as I understand it:
The Good Friday Agreement stipulates there will never be a hard border between NI and ROI.
BREXIT requires a hard border between the UK and the EU.
ROI is part of the EU.
NI is part of the UK.
yes really...
yes, really... really...
There is probably a multitude of business opportunities for some enterprising and very brave soul exporting and importing between these countries.
I couldn't deal with the inevitable stress, or the potential jail time, but that's a 20% additional markup on anything you care to take the trouble to drive over the border as far as I can make out...
I would be shocked if there isn't an incredible amount of technically illegal activity going on!