Hackers accessed phone numbers, email and personal computer details of just under 2,000 subscribers to Fine Gael's revamped website during a four-hour cyber attack, the party has confirmed.
The webpage was allegedly targeted by the "Anonymous" group, best known for its co-ordinated attacks on companies opposed to the whistleblowing website WikiLeaks.
Fine Gael said it had contacted the Office of the Data Protection Commissioner and made a formal complaint to gardai.
A party statement on Monday said: "This morning the party has emailed each of the individuals who submitted their comments and contact data (emails, IP addresses and mobile phone numbers) to the site to notify them of this event. We deeply regret any inconvenience that has been caused to them."
The website, carrying a video message from leader Enda Kenny urging people to share their views on the economic crisis, was attacked between 8pm and midnight on Sunday. It was replaced by a message credited to the Anonymous group referring to censorship.
The Anonymous group is best known for its cyber attacks on companies which it views as anti-WikiLeaks, and has targeted Mastercard, Visa and PayPal.
A spokesman for Data Protection Commissioner Billy Hawkes confirmed an investigation had been launched. "Fine Gael has contacted us about an alleged penetration of their website and we've launched an investigation," he said.
The party said it did not know why it had been targeted, claiming it had received an unprecedented amount of interest in the site over the last week since it was launched.
"The motivation for it seems to be slightly confusing to us in the sense that the entire idea of it (the website) was to listen in a transparent manner, to hear what people have to say," a Fine Gael spokesman said. "So we don't know what their motivation is."
It was taken off-line by the party for a time and a holding page put up as efforts continued to contact those affected.