Users of the online learning portal on the Irish National Teachers Organisation (INTO) website have been warned of a data breach.
The warning said that personal data may have been compromised last week, meaning hackers may have got users' names, email addresses, city, country, gender and other information in connection with their courses.
The website's hacking may have affected around 30,000 users, which includes teachers who took a learning course on the site in the last few years and teachers who had booked retirement planning courses and principals seminars.
The INTO said they have emailed everybody who may have been impacted by the breach.
The union also said the mobile number, school roll number, school post, INTO membership number and Teaching Council registration number in a limited amount of cases may also have been compromised.
They added that hackers did not access any financial information, credit card data or passwords.
Gardaí and the Office of the Data Protection Commissioner were informed of the breach, which is not believed to have been carried out locally.