The attack was discovered on Friday.
Health Service Executive CEO Paul Reid has said the cost of repairing the damage to the HSE's IT system, following the recent cyberattack, could be "tens of millions" of euro.
The cyberattack, which targeted both the HSE and the Department of Health, was discovered last Friday after Department of Health officials noticed unusual activity on Thursday and reported it to the National Cyber Security Agency.
As a result, the HSE was forced to shut down all its IT systems following the ransomware attack, which focussed on accessing data stored in the HSE's central servers.
Speaking on RTÉ's
Morning Ireland on Monday morning, Reid said the HSE will be utilising private hospitals to provide access to oncology services and some diagnostics services.
But speaking on the costs of repairing the system, Reid acknowledged the damage was extensive.
“What we have to do here is a very significant rebuild,” Reid said.
He added: "We can't say at this stage what has been extracted or taken from our system but we do know certain levels of information has been compromised."
Reid described the attack as a "serious criminal act" and said the HSE would do whatever was required to "the greatest extent" to protect its services and patients.
He said he did not know how much money is being sought by the hackers and that this matter had been put "straight into the hands" of the national security teams.
