NI arrangements will hopefully be temporary. The EU is now in decline and the remaining countries will see the UK prosper as they pay more fees into the EU pot. They won't want to stay. ROI will want to make better arrangements with its main trading partner the UK. Also, if NI suffers economically from these arrangements, they will be quick to get them changed. At that time the UK will have no need to do anything the EU wants as we will be on WTO.