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.