There is no definitive answer to this as individual circumstances differ. However, you should endeavour to pay in the currency most suitable to your needs and reach agreement with your supplier accordingly.
If you wish to resell your purchases in Euro, then buying in Euro would probably be most suitable as this would eliminate your foreign exchange risk all together. Effectively, the risk is passed back to the Northern Ireland supplier.
If you have a surplus of Sterling you might wish to pay the Northern Ireland supplier in Sterling.
In all cases, it is important to get the best value for money, so whilst it may be convenient to pay a Northern Ireland supplier in Euro, it is vital that this convenience is not out-weighed by a price disadvantage. It could be more cost efficient to manage the foreign exchange risk yourself.