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 agree a payment currency with your supplier accordingly.
If you wish to sell on your purchases, priced in Sterling, then buying in Sterling would probably be most suitable as this would eliminate your foreign exchange risk all together. (The risk is passed back to the Irish supplier).
If you have a surplus of euro, then you might wish to pay the Irish supplier in euro.
In all cases it is important to get the best value for money, so whilst it may be convenient to pay an Irish supplier in Sterling, it is vital that this convenience is not out weighted by a price disadvantage. It could be more cost efficient to manage the foreign exchange risk yourself if the supplier has loaded his price to cover himself as is often the case.