Two men have been arrested on suspicion of hacking the Fine Gael website.
One man was detained in a raid on his home near Birr, Co Offaly.
A second man was held in the Galway area by fraud squad officers.
Both are being questioned on suspicion of criminal damage.
The investigation was launched in January after a revamped Fine Gael website was broken into and contact details of 2,000 subscribers accessed.
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 site Wikileaks.
The FBI had been called in by American internet firm ElectionMall, which reported the attack to US authorities, to help investigate.
The website was hosted by IT company Rackspace, which is working with ElectionMall.
Fine Gael reported the attack to the Data Protection Commissioner and gardaí after hackers accessed email, phone numbers and computer details of subscribers.
The site had been set up with a video message from Enda Kenny, before he became Taoiseach, urging people to share their views in an open forum on the economic crisis.
The four hour attack saw the homepage replaced by a message credited to the Anonymous group referring to censorship.