One-third of patients at one of Dublin's main hospitals are now infected with the winter vomiting bug.
St Vincent's Hospital confirmed yesterday that approximately 160 of the hospital's almost 500 patients now have the highly contagious bug.
Practically all wards are affected.
Nonetheless, patients are being advised to attend for scheduled appointments unless they have been notified otherwise by the hospital.
In addition to the large number of patients who have contracted the vomiting bug, some 110 staff are now out sick with symptoms, which include diarrhoea and vomiting.
As a result of the worsening situation, the hospital said yesterday its accident and emergency unit was under increased pressure.
Those presenting with minor problems could expect delays, it warned.
It urged the public not to attend the A&E unit unless absolutely necessary and not to visit patients if possible as they risked contracting the bug.
Up to 30 elective operations are expected to be cancelled at the hospital this week.
While the bug can be brought into hospitals from the community, many of the patients currently infected in St Vincent's will have picked up their infection while in the hospital.
Meanwhile, a number of patients at Mayo General Hospital in Castlebar are now also suffering the effects of the winter vomiting bug.
The Western Health Board, which runs the hospital, said two wards were affected and 10 patients were displaying symptoms.
It urged the public not to visit the hospital unless it was essential.
However, it said, admissions are continuing as normal.
An outbreak control committee has been formed and hospital management and staff have put in place measures to reduce in so far as possible the risk of the virus spreading to other patients and to other areas of the hospital, the health board said in a statement.
And it appealed to members of the public with symptoms of viral gastroenteritis to contact their GP rather than presenting at A&E.
Meanwhile, the Irish Nurses Organisation said there were 169 patients being treated on trolleys in accident and emergency units across the State yesterday.