As many others you might be messed up with the currencies used in the Great Britain, Ireland and so on. When going to Dublin you use Euro, but when you decide to visit Belfast it is very useful to know that Sterling, or the British Pound, is the local currency to use. The easiest way to get hold of the local currency is to use an ATM anywhere in the city, near your hotel or maybe at Belfast Airport. The abbreviation for British Pounds is GBP,.
Exhcange rates (as of July 2012):
100 GBP = 157 USD
100 GBP = 127 Euro
These rates change quite often, so if you want to be up to date, use the currency converter instead.