Data linked to about 520 patients was posted online following the recent cyberattack, the HSE has confirmed.
In a statement, the health service said the data contains correspondence with patients, minutes of meetings and includes sensitive patient data.
The HSE also confirmed corporate documents are among the illegally accessed information.
A criminal investigation is underway into the breach and the HSE said it was working with An Garda Síochána.
The HSE said a “news publication” recently wrote a story saying it had seen HSE data which had been illegally accessed.
“We informed the publication of the court order we obtained in relation to this matter last week and asked them to supply it to us, and they agreed.
“We have examined it and can confirm it is HSE data relating to approx 520 patients, as well as some corporate documents. The data includes sensitive patient data, minutes of meetings and correspondence with patients.
The HSE’s Data Protection Office has followed the appropriate procedures, including notifying the relevant health service providers and the Data Protection Commission, it said.
“The process of notifying the patients involved has commenced. This will involve some further analysis of the data, and we will do this as quickly as possible.”
The HSE said it was not aware at this stage of any further publication of data.
Cost to health service
On Thursday HSE boss Paul Reid said the total cost of the cyberattack would cost in excess of €100 million.
Up to 7,000 outpatient appointments a day are being lost as a result of the attack by a criminal gang that has played havoc with the health service, the chief executive said.
Speaking on Thursday, Mr Reid said €100 million would be “a small enough figure” when it comes to the total cost of dealing with the attack.
He said: “There’s a number of costs that we will incur by purely getting the systems back up, and secondly, upgrading some systems during this process.
“There’s a resource cost with all of that.
“Then there will be the costs based on our services and the impact on trying to recover services.
“I said at the outset this will be in the tens of millions.
“And there’s no doubt that 100 million would be a small enough figure in terms of the total costs of this.”
HSE chief operations officer Anne O’Connor said that outpatient services are currently operating at about 50 per cent capacity, meaning around 7,000 appointments a day are being lost.
But she said this is only one area affected, and it will be some time before the impact across all patient groups is clear.