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 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.