Afternoon,
US customs, immigration and Homeland security are sited at Dublin and Shannon. All the prechecks are done before boarding, plus Homeland Security fingerprint and iris checks.
Flew to NY in October from Dublin and it worked like a dream. On arrival in NY picked up the bags and straight out the door!
Under the visa waiver scheme a Visa is not required for Irish citizens provided they stay in US for no more than 3 months. As far as I know this applies to UK citizens aswell, but I don't know if US customs, immigration etc. are sited at any UK airports. I'd be interested to know.
Your Foreign Office website should be able to give more details.
Hope that helps,
Lz