If an Irish citizen travels from Ireland to America as a tourist, for a week, do they need a visa?
The Visa Waiver Program "allows citizens of participating countries* to travel to the United States without a visa for stays of 90 days or less".
(Ireland IS a participating country).
Each traveller must have authorisation under ESTA.
It costs US$14, and the US Embassy in Dublin has all the information you would require.
It takes up to 3 days to process, is valid for 2 years, and you need to have it approved before you leave for the US. It's all done online. ( - and is fairly simple - I've done it twice before, myself)