I know it might sound strange, but currency is an important issue. There have been people I welcomed in Dublin who arrived in Ireland with Sterling or asked me whether it was better to exchange Euros for Pounds at the airport or in the city center.
Of course, mistakes happen, and we can laugh about it, but changing money twice is not cost-effective. It’s better to bring the exact currency with you, or, alternatively, some cool sesterces if you’ve got them.
To sum it up: in Ireland, the currency used is the Euro. However, if you plan to take a trip to Northern Ireland, you’ll need to exchange your Euros for Pounds.
A side note: If you’re thinking of visiting Germany, be aware that you’re probably aiming for Berlin, not Dublin… because this has actually happened too!