It’s really the Irish Republic that is claiming it has concerns, over a small amount of possible tariff revenue loss and its ability to sell its agricultural products to the EU.
Ireland is not part of the UK and will not be exiting the EU with the UK. Truth is the EU has brought about a nearly United Ireland that people on both sides have gotten very used to. Irish products made in the ROI and NI go back and forth across the border much like products move between LA County and Orange County.
there are no Tariffs between EU countries. However, there will be once again with Brexit.
NO HARD BORDER