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 webpage -- 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 US internet firm ElectionMall, which reported the cyber attack to US authorities, informed the party that the FBI was now involved.
Fine Gael said it was working with ElectionMall but had not requested an FBI investigation. 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.
During the attack on Sunday the website was replaced by a message credited to the Anonymous group referring to censorship.
A Fine Gael statement said: "The party has emailed each of the individuals who submitted their comments and contact data to the site"