The FBI has been called in after hackers broke into Fine Gael's revamped website and accessed contact details of almost 2,000 subscribers.
The site - hosted by a US company - was allegedly targeted by the 'Anonymous' group, best known for its co-ordinated attacks on companies opposed to the whistleblowing website WikiLeaks.
A Fine Gael spokesman said American internet firm ElectionMall, which reported the cyber attack to US authorities, informed the party the FBI were now involved.
"That was their information," the spokesman said.
He said the website was hosted by IT company Rackspace, which is working with ElectionMall.
He said it was normal protocol between the two companies that in the event of a cyber attack they would make out a report to be submitted to the FBI.
Fine Gael said it was working with ElectionMall but had not requested an FBI investigation or had any contact with the US agency.
The party reported the attack to the Data Protection Commissioner and gardai after hackers gained access to email, phone numbers and computer details of subscribers to the website, which carries a video message from leader Enda Kenny urging people to share their views on the economic crisis.
Fine Gael's new website was set as an open forum for the public to air opinions.
During the attack between 8pm and midnight on Sunday the website was replaced by a message credited to the Anonymous group referring to censorship.