The UL Hospitals Group is writing to 630 patients over a breach of their data at University Hospital Limerick.
In a statement released today, the UL Hospitals Group said the data breach relates to patients who attended the Emergency Department at UHL between 18 April and 22 April.
"The data in question was extracted from an automated system used in the ED to dispense medication safely. It was extracted, without HSE knowledge or approval, by an employee of a company which was then supporting this system; and not by any employee of the HSE," said the statement.
"This information was published online in the form of a file linked from a Twitter account."
The file contained patients' names, dates of birth and the names of medications dispensed while they were in the ED.
However the statement said that the medications "were for the most part those you would expect to be dispensed in an emergency department (i.e painkillers and antibiotics)".
The UL Hospitals Group said they became aware of the breach on 29 May.
"Immediate actions were taken by the HSE and by UL Hospitals Group to protect patient data. Twitter blocked the link to the data and disabled the account in question.
"An Garda Síochána were notified and the HSE obtained a High Court Order on 5 June restraining the individual concerned from communicating confidential information. This breach was also reported to the Data Protection Commission on 29 May."
The Group said it was "now writing to patients to comply with data protection regulations and to advise that there remains a residual risk of future unauthorised disclosure, in spite of the High Court injunction that remains in place to restrain the individual from further sharing data".
Where the patients concerned are children, the Group is writing to the parents or guardians. Of the 630 patients involved, 95 are children.
"We have apologised to our patients in writing for this data breach and for any distress this will cause. We have also set up a helpline and shared these details with the patients concerned.
"UL Hospitals Group has also convened a Serious Incident Management Team (SIMT) to investigate this incident at a local level and take any necessary actions to further secure patient data," the statement added.