The Health Service Executive (HSE) has confirmed that the data of around 520 patients has emerged online following a cyber-attack earlier this month.
In a statement, the executive says sensitive information about patients has been shared, along with minutes of meetings.
The HSE is now working to identify the patients involved and says it will work to inform them as soon as possible.
This update relates to a leak that was previously reported by the Financial Times.
However the HSE says it is not aware of any further attempts to publish the data.
It says it informed the Financial Times of a court order obtained last week, and asked them to supply the information, which they agreed.
A HSE statement says: "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.
"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.
"This data was the initial small tranche of data that was previously reported on, and we are not aware of any further attempted publication of our data.
"We apologise for the inconvenience caused to our patients and service users. The HSE is working with An Garda Síochána on this criminal investigation," it adds.
Following the attack, the Russian ambassador to Ireland Yury Filatov said his country had offered to conduct a joint investigation.
He said he has no way of judging who was behind the attack – but noted that Russia is targeted by hackers "on a daily basis."
He said Russia has a "real interest in promoting international cooperation" on cybercrime and was offering to investigate the attack jointly.
Additional reporting: Jess Kelly