The HSE says it will contact 113,000 people whose details were accessed during last year's cyber attack.
The health service was targeted by a major ransomware attack in May of last year.
The HSE says 80% of its IT systems were impacted.
National Director of Operational Performance and Integration with the HSE, Joe Ryan, explains what type of personal data was accessed.
"It's important to know that the core systems in the HSE were not compromised in terms of data being taken from them, so not entire medical files.
"The vast majority of the data was essentially lists, names and contact details. It could be a mobile number or it could be an email address, it could be an address.
"There is a small amount of clinical data that would be in the form of referral letters."
