Up to 150 patients and staff at hospitals across the State have fallen ill with the winter vomiting bug.
Worst hit is Dublin's St Vincent's Hospital where the bug has caused major disruption. Fifteen wards have been closed to new admissions. The hospital confirmed yesterday that approximately 76 patients and 43 staff were showing symptoms.
"As a result, all elective admissions are being risk-assessed. There have been five surgical cancellations," the hospital said.
"The winter vomiting virus is in the community generally and is being brought into hospitals where it is having a serious, disruptive effect," it added.
Hospitals in Galway, Limerick, Kilkenny and Carlow also said they had a small number of cases. So too has Beaumont Hospital in Dublin. University College Hospital Galway said it has closed one ward where four patients had symptoms of the bug to new admissions.
"We have a number of other patients throughout the hospital, with symptoms of diarrhoea and vomiting, community acquired, which are isolated," it added.
A spokesman for the Mid-Western Health Board said two patients were displaying symptoms at the Mid-Western Regional Hospital in Limerick.
The South Eastern Health Board also confirmed there were a number of cases at two geriatric hospitals in its region. It said there were five cases at St Columba's Hospital in Thomastown and one case at the Sacred Heart Hospital in Carlow, which is closed to visitors and admissions.
The Southern Health Board, Midland Health Board, North Eastern Health Board and North Western Health Boards said they had no cases.
All health boards and hospitals with cases appealed to the public for assistance to prevent the bug, which is highly contagious.
Mr Nicholas Jermyn, chief executive of St Vincent's Hospital, said the current outbreak was "serious" and he anticipated more cases would occur before the outbreak peaked. The hospital would have to seriously consider cancelling more surgery, he said.
"We have 75 patients now with it and we have over 40 staff out sick and 15 wards affected. The impact is that obviously it affects the accident and emergency department and many other departments," he said.
"We would anticipate over the next five to seven days it will peak and hopefully it will then start to run down and we will be able to see a recovery situation."
He claimed the bug was being brought into the hospital by visitors. He urged people not to visit relatives in hospital if they had been in contact with the winter vomiting bug.
The Eastern Regional Health Authority advised anyone with symptoms not to visit A&Es as they risked spreading it to others. Instead they should stay at home, drink plenty of fluids and limit contact with others.
Winter vomiting is usually a mild illness which lasts for about two days. If symptoms persist people should contact their GP preferably by phone first, instead of turning up in the surgery where they risked passing the virus on to others, the ERHA said.
Meanwhile, the Irish Nurses Organisation said 164 patients had to be treated on trolleys in accident and emergency units across the State yesterday.
In Dublin St Vincent's and Tallaght Hospitals were worst affected. Outside of the capital, Cork University Hospital had the largest number on trolleys, where the INO said there were 20 patients on trolleys in its A&E unit.
And following reports that six deaths at Dublin's Mater hospital last year were linked to the Clostridium difficile superbug, the Department of Health has admitted the bacterium was not notifiable and no database was kept on the number of cases.