The Health Service Executive (HSE) will start notifying 113,000 people whose information was illegally accessed during a cyber attack.
Patients and staff who had their personal information stolen and copied last year will be told by letter.
The HSE said the process will take until next April because of the number of victims.
The ransomware attack resulted in the HSE having to close down its IT services, causing widespread delays and the cancellation of appointments at hospitals across the country.
Personal details including names, addresses, phone numbers and email addresses were taken.
Medical information, including notes and correspondence, lists of patients receiving treatment and vaccination details, was also stolen.
Some 84 per cent of the victims were patients.
Joe Ryan, the HSE’s national director leading the notification programme, said: “As a result of our extensive monitoring and support from security services, we have seen no evidence that personal data relating to the HSE cyberattack has been shared or used fraudulently.
“We are very sorry that this occurred and ask for people’s understanding as we work through this complex administrative process, in which we hope to support people and continue to answer their questions and requests.
“This notification process is an important duty for the HSE, as we held people’s personal data, and through this cyberattack on HSE systems that information was compromised.”
The HSE said it will be apologising to those affected, who will be told which documents were stolen and how they can see them.
Mr Ryan added: “The notification process will go on over the coming weeks and months as we have to take great care in notifying people correctly and securely.
“The first group being notified includes approximately 850 HSE staff members.
“We are writing to them to notify them that data relating to their staff travel expense claims was illegally accessed and copied.
“This data contained some limited financial details.”
The compromise was classed by bank officials as low risk, he said.
He added: “We expect the notification process will take a number of months to complete as we take the time to contact each person, ensure we have a secure communication with them and go through the process of assisting them if they want to make a request to view their documents.
“Over the coming months, we will be writing to approximately 94,800 patients and around 18,200 members of staff.”
The HSE said it “sincerely regretted” the cyberattack’s impact and has taken a “thorough approach” in responding to it.
Specialist security partners of the HSE have been monitoring the internet, including the dark web, for signs of the data surfacing.
“We also engaged with the large digital publishers, the search engines and social media networks,” Mr Ryan said.
“We’ve continued to engage with them in our efforts to ensure that there’s no movement of the data or any false information pertaining to it.”
Due to systems shared with the HSE at the time of the cyberattack, Tusla and Children’s Health Ireland were also impacted.
Both bodies will be telling people of their respective processes.