Beaumont Hospital in Dublin has restricted visiting following an outbreak of the winter vomiting bug.
The hospital said the outbreak has spread rapaidly since the end of last week and affects the majority of wards.
As of lunchtime today, around 40 patients were displaying symptoms of the virus.
Up to 13 of these were new cases since yesterday and 28 beds have been closed in the wards since yesterday.
The movement of staff between wards has also been restricted.