"Overview of Northern Ireland Trade"
From the BBC:
A No 10 source says a Brexit deal is "essentially impossible" after a call between the PM and Angela Merkel... They also claimed she said a deal would never be possible unless Northern Ireland stayed in a customs union.
I'm all in favour of a (re)united Ireland and NI remaining in the EU customs union is a step in that direction. I'm not sure why the EU is so adamant, but it's not necessarily A Bad Thing.
It depends largely on whether NI trades more with RoI ...