The HSE said the decision had been taken as a “precaution” and that Covid-19 vaccination appointments were not affected by the attack.
Former Taoiseach Leo Varadkar said that “international criminals” appeared to be behind the attack, adding that the impacts could continue into next week, while junior minister Ossian Smyth labelled the incident “possibly the most significant cybercrime attack on the Irish State”.
In a tweet, HSE Ireland said: ”There is a significant ransomware attack on the HSE IT systems. We have taken the precaution of shutting down all our IT systems in order to protect them from this attack and to allow us fully assess the situation with our own security partners.
“We apologise for inconvenience caused to patients and to the public and will give further information as it becomes available.
“Vaccinations not effected are going ahead as planned.”
It added that the national ambulance service continues to function normally with no impact on call handling or dispatch.
HSE boss Paul Reid told RTE’s Morning Ireland: “Our first priority is to contain the issue. This is a major incident for us. As the morning progressed we’ll gain greater clarity on the impact of this.”
Hospital equipment has not been affected by the attack, which he said is an IT system issue.
Mr Reid encouraged people to continue to attend vaccination appointments.
The HSE is being supported by the government, cyber security experts and the Gardaí and the defence forces.
Although vaccinations have not been affected, the Rotunda maternity hospitality in Dublin has been forced to cancel appointments due to the attack.
The hospital said that all outpatient visits are cancelled on Friday, except for patients who are 36 weeks pregnant or beyond. All gynaecological clinical have been cancelled for the day.
It advised that people with “urgent concerns” should attend as normal.
Founded in 1745, the Rotunda hospital is the oldest continuously operating maternity hospital in the world.