Fourteen patients and 16 members of staff at St Patrick's Hospital in Waterford city are being treated for the Winter Vomiting Virus.
However, the hospital is open for restricted visiting, and visitors are asked to contact the hospital before coming.
The board has asked the public not to visit the hospital if they have any symptoms of the virus, such as diarrhoea, vomiting or cramps.
There has been one suspected case of the virus among patients since Monday.