The Western Health Board has confirmed that 25 people were treated followed an outbreak of the SRS virus in a hotel in Westport, Co Mayo, last Friday.
Westport Woods Hotel, part of the McEniff group, is not accepting residents at present but is open for all other business.
Residents were offered alternative accommodation when the virus was detected.
Some of them opted to stay on in the hotel.
The virus - known as the winter vomiting bug - has affected several hospitals throughout the State over the past year. Dr Diarmuid O'Donovan, public heath specialist with the board, called the virus a "nuisance illness".
"The hotel is not at fault at all," he said. "It is likely that the virus was brought in by someone coming in."
He added that the virus was highly contagious and that Mayo General Hospital in Castlebar was one of the hospitals affected in the region.
A team of doctors and environmental health officers has been sent to deal with the outbreak.
The bug usually lasts two to three days and is similar to gastric flu.
Incubation can takes a few days although a person can be a carrier for some time afterwards.
A health board source said there was documentary evidence that the virus was a growing problem in the hospitality industry or in hospitals.
"It can affect the best-run in the country," the source added.
The manager of the hotel, Mr Michael Lennon, said the outbreak was "one of those things we can do nothing about". He plans to open the hotel to residents again on Friday.