The HSE has shut down its entire IT system after a major cyber attack.
The Covid vaccination programme isn't affected - but the Rotunda Hospital has cancelled most appointments for today.
Holles Street says there will be a significant disruption to all services.
The HSE has confirmed there's been a "significant ransomware attack" on its IT systems - and has shut down the entire network to protect them.
It's apologised for the inconvenience caused to patients and the public.
Covid vaccinations aren't affected and are going ahead as planned.
The Rotunda has cancelled all appointments for today - except for women who are more than 36 weeks pregnant.
It's urging people not to attend the hospital unless it's an emergency.
While Holles Street says there will be significant disruption to all services but people with appointments are being told to come as normal.