The Irish National Teachers Organisation has warned users of its online learning portal that their personal data may have been compromised following the hacking of the website.
The breach, which happened last week, left names, email addresses, city, country, gender and information related to course actions potentially accessible by the hackers.
The INTO said around 30,000 users details were potentially compromised by the hack.
These were teachers who did online learning courses in the last few years, as well as teachers who had booked retirement planning courses and principals seminars.
The union said all concerned have been informed.
In an email to those impacted, the union also said that in a limited amount of cases mobile number, school roll number, role in school, INTO membership number and Teaching Council registration number may also have been compromised.
However, the primary teaching union says no financial information, credit card data or passwords were accessed by the hackers.
The Office of the Data Protection Commissioner and gardaí have been informed of the hack, which is not thought to have been carried out locally.
INTO Learning is the professional development and union training section of the union.
It offers a range of online and in person training courses for members around the country.
This breach relates to its website and not the main web pages of the union.
The INTO said it takes security, data protection and privacy issues seriously and while it has no evidence to suggest this data was stolen, the data was "potentially at risk and thus potentially accessible to the third party behind the breach."
The union says early enquiries suggest the breach involved the hackers using the server on which the data was stored as a base to send spam messages.
The website has been taken down as a precaution and a full report is awaited from the company responsible for managing the website, which will be followed by an investigation.
However, the union says when it is up and running again, users will have to change their passwords as a precaution.