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.
New Report Warns Of “Poor Liquidity” As Ireland’s Housing Market Stalls
Laois TD Renews Calls For Updated Wind Farm Guidelines
Midlands Man To Be Laid To Rest Today
Anti-Racism Funding Granted For Offaly And Westmeath Groups
Justice Minister Wants People To Come Forward Over Offaly Arson Attack
Gardaí Begin Murder Investigation After Child And Woman Dies In Offaly House Fire
Ashling Murphy Murder "Wrongly Accused" Granted Bail Over Theft Charge
Young Boy And Woman Killed In Offaly House Fire
Laois Makeup Artist Warns Of Expired Suncream Dangers
Lynx and Wolves In Ireland Pose Minimal Threat - Offaly Biodiversity
€4.5m Going To Midlands Suckler Farmers
Westmeath Writer Nominated Posthumously For Award
Seven Midlands Hotels Nominated For Gold Medal Hotel Awards
Cabinet Approves Major Expansion Of Midlands Garda HQ And Prison
Memorial Day Honouring Ashling Murphy Takes Place In Offaly
Midlands Town Left Shocked After Death Of Young Woman
Concerns Highlighted On Drones Spotted Ahead Of Zelenskyy Visit
€6m Westmeath Lotto Jackpot Winner Comes Forward
Israel’s Participation Sparks Eurovision Withdrawals From Ireland And Others
Teachers Union Calls For More Permanent Posts Amid Recruitment Challenges