The HSE says its IT systems have been hit with a 'significant' cyberattack.
It has shut down its systems as a precaution, with HSE CEO Paul Reid describing the ransomware attack as "quite sophisticated and very targeted".
COVID-19 vaccination appointments are not impacted and are going ahead as planned.
The National Ambulance Service is also operating as normal, with emergency calls still being handled.
A ransomware attack is one in which hackers threaten to publish data or block access to computers unless a 'ransom' is paid.
In a statement, the HSE 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."
The Rotunda Hospital says it has cancelled most appointments for today, except for emergencies.
Rotunda Critical Emergency
Due to a serious IT issue all outpatient visits are cancelled today - unless you are at 36 weeks pregnant or later. All gynae clinics are cancelled today.
If you have any urgent concerns please attend as normal.
Further updates will follow.
— The Rotunda Hospital (@RotundaHospital) May 14, 2021
The National Maternity Hospital has also warned of "significant disruption", although is asking those with an appointment or those who need to attend to come as normal.
On Newstalk Breakfast, HSE CEO Paul Reid said the cyberattack is "quite sophisticated" and "a very targeted attack on national infrastructure".
He said: "It seeks to get access to data immediately, so as a precautionary measure... we've had to shut down a lot of our major national and local systems to protect them.
"Apologies to everybody for the impacts, but we've taken the appropriate actions immediately and swiftly."
Mr Reid said a crisis management team has been meeting, and they're assessing the impact on services.
He said patients should carry on as normal "unless and until people hear from us" - but says the incident will impact on services in a number of hospitals later in the day.
He noted that the COVID-19 vaccination programme is on a separate system so isn't impacted.
The Tánaiste Leo Varadkar has said while vaccinations are unaffected, the system offering people COVID-19 tests has been hit.
"It is a situation that's still evolving, there's lots that we don't know.
"But it appears to be a ransomware attack by international criminals, essentially, and the problem could run through the weekend and into next week unfortunately.
"What I can say is that emergency services, ambulance services, GP systems and pharmacy systems are not affected.
"And also the vaccine programme will not be affected either.
"Unfortunately the testing programme will be - not the testing itself, but giving people appointments for the tests - so that is definitely a difficulty for the next couple of days.
"And hospital systems are affected; because the HSE has had to shut them down to make sure they're safe before they can turn them on again".
Newstalk's technology correspondent Jess Kelly says the incident is very worrying.
She said: "Something like the HSE... would have extensive records and very sensitive data.
"This is a very serious issue. The HSE says it has locked down the system to try to deal with this problem, but it is absolutely a headache this morning."
Additional reporting: Jack Quann
Main image: File photo. RollingNews.ie