It's apologised for the inconvenience caused to patients and the public.
The head of the HSE says its IT system has been the subject of a major, sophisticated cyber attack.
The entire network has been shut down - but the Covid vaccination programme isn't affected.
The Rotunda Hospital has already cancelled most appointments, except for women who are more than 36 weeks pregnant.
The National Ambulance Service is operating as normal, with emergency calls still being handled.
Paul Reid says other services will be affected too - but people should attend their appointment unless they hear otherwise:
Technology correspondent Jess Kelly says the incident is very worrying:
Child and family agency Tusla says its systems aren't operating because of the HSE cyber attack.
It includes the portal that's used to make child protection referrals.
Anyone wanting to make a referral can contact their local Tusla office.