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, has informed the party that the FBI is now involved.
The office of Data Protection Commissioner Billy Hawkes is investigating here, while Fine Gael has also contacted the Garda Computer Crime Unit about the incident.
In a statement, Fine Gael said it had this morning emailed all of those affected.
The Fine Gael website was forced offline last night after hackers attacked it removing the content and posting a message referring to censorship by the political party.
Fine Gael rebranded its main website, finegael.ie, last week as finegael2011.com and invited members of the public to post comments and register their mobile numbers and email addresses to receive campaign messages.
An Evening Herald journalist claimed on social networking website Twitter that he had been sent the details of 4,000 users of the Fine Gael website following the attack.