Two Dublin hospitals have requested that members of the public only visit them when essential following an outbreak of the winter vomiting virus.
Beaumont and St Vincent's hospitals said the virus had already affected some 100 staff and patients.
A spokesman for Beaumont hospital said some 40 beds were unavailable as a result of quarantine arrangements put in place to deal with the outbreak.
He said "only truly essential visits should be made" and that there should be no more than one visitor per patient.
"The outbreak is being managed appropriately," a hospital spokesman said. "So far we have refrained from imposing an all out ban on visiting…but we may have to do that if people do not fully respect the existing restrictions."
St Vincent's said it had identified 59 cases of the virus and that 19 beds had been closed as part of its effort to curb the spread of the illness.
"St Vincent's is appealing to the public to attend the emergency department only if necessary and not to bring children to the hospital," a spokesman said.
"Members of the public who have symptoms such as 'upset tummy' should not attend the hospital but contact their GP in the first instance if they have serious concerns."
Symptoms of the virus include abdominal cramps, vomiting and diarrhoea.
Dr Ed Smyth, consultant microbiologist at Beaumont hospital, said the outbreak was one of the worst seen in recent years.
He said a total of 153 patients and staff had shown symptoms of the virus at Beaumont since the start of the month and there were 45 patients being treated in the hospital for the illness today, with 30 new cases emerging overnight.
"We really are quite concerned about it and despite taking all the appropriate measures the outbreak continues," he told RTÉ News at One.
"I think part of the problem is that there is a good deal of norovirus in the community around the hospital. We have patients and more importantly families and visitors bringing it into the hospital with them."