Strict visiting restrictions have been introduced at the Midland Regional Hospital in Mullingar to try to stem the spread of the "winter vomiting bug" which has afflicted up to 50 people at the hospital.
Only those visiting seriously-ill patients are being allowed access to the 200-bed hospital, which has cancelled all elective admissions and surgery because of the outbreak.
Two other hospitals in the west and north-east have confirmed cases of the bug, while the Eastern Regional Health Authority (ERHA) said there were almost 40 new cases at a residential unit in the East Coast Health Board area.
A spokeswoman for the Midlands Health Board said hospital staff were among those infected at Mullingar since the outbreak two weeks ago. She said the hospital had between five and six cases at present, all of who were confined to a particular ward.
"The situation is changing every 48 hours."
Of the new access restrictions, she said: "We have stopped all visitors coming and going from the hospital except those visiting seriously-ill patients."
As the bug was not a notifiable disease, she added, it was difficult to establish the precise number of infections in the health board area.
"People in the community who might have it would be going to their GPs rather than coming to hospital. There have been rumours of cases elsewhere but no more confirmed cases."
Primary symptoms of the gastroenteritis virus, known as SRSV which is most common during winter, are diarrhoea and vomiting.
Some people also suffer from a raised temperature, headaches and aching limbs. The effects tends to last between two and three days.
A spokeswoman for the ERHA said a number of recent infections at Beaumont Hospital and a crèche in Tallaght, Dublin, had cleared up.
However, about 24 clients and 14 staff at a residential centre for elderly people with intellectual disabilities in the south Co Dublin/Co Wicklow area had fallen ill with the bug.
"We have not had anyone with a serious illness. This is an illness which crops up every winter, and while it is unpleasant it is self-limiting in that it tends to clear up after a couple of days."
The two other hospitals affected are Cavan General and Roscommon County, where four and three cases respectively have been confirmed. Visitors to all affected hospitals have been advised not to bring children or exposure-prone food, such as fruit, with them.
Other health boards said they had no confirmed cases of the virus.
The outbreak is believed to have originated in Northern Ireland in December. Since then, hundreds of cases have been reported in Britain, including more than 200 at Glasgow's Victoria Infirmary Hospital last month.
