A TWITTER account called Anonymous Sweden has claimed responsibility for the hacking attacks on two government websites.
It said in a tweet that the attack was carried out in protest against planned new copyright legislation in Ireland. Two Government websites were forced offline for several hours overnight in co-ordinated cyber attacks.
A second Twitter account named AnonOps also declared the justice website closed late last night before publishing a link to a web page carrying a list of every TD's email address.
Around the same time, a third account, named YourAnonNews, published a link to a page carrying the phone number of every TD and then declared the finance website shut.
Later tweets denied claims that two other Government-related websites had been targeted - the Blue Blindfold anti-human trafficking site and the Freedom of Information site.
Some 29,000 signatories have backed an online petition against plans to allow music publishers to take internet service providers to court to protect their material from file-sharing and online piracy.
The Government believes that a 2010 High Court ruling - in a case brought by music giant EMI against broadband supplier UPC over illegal downloading - means there is a legal loophole which needs to be closed.
The Government has described the incident as a distributed denial of service, where huge volumes of internet traffic were directed to the websites.
The attacks are believed to be co-ordinated and a response to the looming reform of copyright laws, dubbed the Irish Stop Online Piracy Act (Sopa), which is due to be published in the coming days.
A spokeswoman for the Department of Justice said: "This was not an attempt to extract information from the website but was instead an attempt to stop the service.
"There seems to be no damage done to the website. However, a review of the website is being conducted this morning.
"The situation continues to be monitored by the Department of Justice and the Department of Communications. The Government is aware of the potential threat of this type of cyber attack and the Department of Communications is also readying a whole of Government response to this threat."
A Department of Finance spokesman said IT staff were alerted to the threat of an unusually high flow of traffic to the website and, as a failsafe, shut the website down.
He said there was no hacking attempt because no one gained access to the site. It was merely an attempt to disrupt service.
"There was an inordinate amount of traffic and data hitting the website last night," said the spokesman.
"The system sends an alert to IT and it should investigate it as there is a built-in failsafe.
"It wasn't a hacking-in that a hacker needs to get into the website. We would suspect that somebody was trying to create some sort of mischief, but IT were alerted to the irregular flow of traffic and shut it down themselves."
Sinn Fein communications spokesman Martin Ferris claimed the Government had been vague to date on the implications of tightening copyright rules for the internet.
"Experts in the field are taking a diametrically opposed view to that of the minister (Sean Sherlock)," he said.
"There is understandable fear among both commercial and private users of the internet that this will leave the door open to actions that would severely restrict access to information if copyright holders can secure orders preventing it being made available.
"That would not only represent a significant attack on individual freedom but also a threat to a healthy sector of the Irish economy which has attracted major international players to locate here."