There are two currencies used in Ireland depending on what part of Ireland you are visiting:
Northern Ireland and the Republic of Ireland have different currencies. In the Republic of Ireland the currency used is the Euro and in Northern Ireland the currency used is the Pound Sterling. The banks in Northern Ireland all issue their own notes and with this you may come across different variations in the notes. All coins are the same, irrespective of the bank from which they come.
When crossing between the Republic of Ireland and Northern Ireland you can exchange your money in shops, gas stations, bureaux de change and banks. It is also wise to check the exchange rate when changing money.
ATMs are present in very town in Ireland so therefore are a convenient way of dealing with your money during your stay.
Below are various links to pages in which you can discover more about the notes and coins of Sterling and the Euro: