Very simply, Irelands electricity supply is 220 volts at 50hz, whereas the United States uses 120 volts at 60hz. Converters and sets of adapter plugs are available at travel & luggage stores and at Radio Shack/Tandy and other electronic stores. They can often be found in airport shops and duty free stores. A set of adapter plugs costs around $10 to $15US and in some stores you can buy an individual adapter for only a few dollars.

Ireland uses a rectangular blade plug, with a ground pin. It looks like this: