GB sell to ROI deliver to NI

Hi, we are a UK company, if we sell to a Irish (ROI) customer but deliver to a Northern Ireland ship-to customer of theirs who is liable for duty please and would there be duty if the goods are remaining in NI?

Hi sarah Johnson,
There may be duties due depending on the origin of the goods and whether they are ‘at risk’ of onward movement to the EU. 
Please see the guidance at:
Moving goods you bring into Northern Ireland as 'not at risk’ of moving to the EU
Thank you.