A man has been arrested after a list containing personal details of more than 500,000 GAA members was stolen.
It was taken from Servasport, a Belfast-based company that develops and maintains the database.
It is understood that a major line of enquiry is that a disgruntled former employee copied the information.
It was subsequently posted to the Gaelic Players' Association, the Data Protection Commissioner and the Information Commissioner.
In total, the list contained the names and addresses of 501,786 members.
The GAA (Gaelic Athletic Association) said that due to an ongoing investigation by the PSNI, it had been unable to inform its members until today.
It is understood the man arrested has been released on bail.
Servasport said it had apologised to the GAA.
As well as the names and addresses, the database holds:
- 288,511 dates of birth
- 107,212 mobile numbers
- 63,695 landline numbers
- 30,171 email addresses
In 544 individual cases, the database referred to a medical condition.
The GAA said it would write directly to each of these members setting out the information about them.
Over 167,000 of the members on the database are under 18.
An association spokeswoman added that no other GAA IT system had been affected by the incident and stressed that the database did not contain any financial information.
She said that a business consultancy firm had been asked to undertake an independent review of Servasport.
"This matter is being taken very seriously," the spokeswoman added.
Servasport said in a statement that it was assisting the police and the Data Protection Commissioner with their investigations.
The statement added: "We understand that the PSNI has good made progress with its investigations and we are very hopeful that this matter can be resolved speedily and with no misuse of the information ever taking place."