Boards.ie has gone offline as a result of the attack
Irish internet forum Boards.ie has reported an attack on its user database which could affect thousands of users.
In a statement it confirmed that "an unauthorised source" from outside Ireland accessed its database server.
It urged members who use the same username/email and password on other sites to change passwords.
"Part of the database which includes our members' usernames, email addresses and obfuscated passwords was accessed," Boards.ie said on its homepage.
The site started life as a forum for the computer game Quake in 1998 and has more than 500 forums on a range of topics.
According to the most recent ABC internet traffic statistics in November, Boards.ie had more than 20m page views, averaging more than 1.1m page views a day.
Tom Murphy, co-founder of Boards.ie said, in a statement, that like all large sites they were "regularly the target for disruption and take continual actions to proactively protect data".
He said that the attack "was completely unprecedented despite our rigorous security measures and while we have no idea if this data will be used for any malicious reasons, we felt it vital to tell you this immediately".
Boards.ie says it is changing all user passwords and has notified Irish police, the Data Protection Commissioner Billy Hawkes and an independent security consultant.
Mr Hawkes said the attack should be taken as a warning to internet users to create different passwords for different websites.
He said his office would be investigating the security breach but he praised the company for pulling the website immediately and alerting its members.
The site has been posting updates on Twitter, with a tweet telling users Boards.ie would "hopefully" be back on Friday morning.