Tallaght Hospital in Dublin has admitted that patient medical records have been the subject of unauthorised access and disclosure.
In a letter to hospital consultants on outsourcing the transcription of patient medical reports and letters for GPs, the hospital has also conceded that while it was the policy not to identify patients, this policy has not always been followed.
On 23 July, RTÉ News revealed the hospital had reported a potential data security breach to the Data Protection Commissioner, however afterwards the hospital said these were 'unsubstantiated allegations' and that it had no evidence of disclosure.
Since 2004, the hospital has been using a private firm to transcribe some medical reports and letters for GPs.
That firm was sending the material to its offices in the Philippines for transcription.
Tallaght Hospital says some letters that were dictated did not come back transcribed and while it was hospital policy to keep information sheets for each letter, this practice was not always followed.
Minister for Health Dr James Reilly has expressed concern at the emerging information regarding the unauthorised disclosure.
In a statement, the minister said he hoped the Data Protection Commissioner's investigation can be completed as quickly as possible.
Dr Reilly has called on the hospital to provide information to concerned patients and had asked the HSE to establish if any of their hospitals are similarly affected.
The hospital is now setting up a unit to respond to patient queries.
Tallaght says it is seeking the help of the National Bureau of Investigations in the Philippines and the UK Information Commissioner.
It is trying to ensure that all data is returned to the hospital. Its director of information technology is in the Philippines working with the authorities on the patient data security breach.
Tallaght Hospital says the contract with the firm ended and it has moved to a new provider.
However, it has warned consultants that all clinicians that use the transcription service must assume that some information on patients is being disclosed.
In its letter to consultants, the hospital says the scope of the issue is much larger than Tallaght Hospital.
Tallaght Hospital is a voluntary hospital funded by the Health Service Executive but not run by it.
The HSE has said none of its hospitals outsource the typing of medical reports or letters for GPs.
Tallaght Hospital says that patients concerned with possible breaches of confidentiality may contact the hospital free phone inquiry line 1800 283 059.
It will be in place between 9am and 5pm from tomorrow.