Management at Sligo General Hospital has implemented an infection-control plan following the confirmation of 12 cases of the winter vomiting virus at the hospital.
It is feared that the numbers affected could rise in the coming days, and the results of laboratory tests on at least eight more suspected cases are expected today.
According to the North Western Health Board (NWHB), five wards at the hospital have already had cases of the virus. As part of the infection-control plan, admissions to these wards have been suspended, while the discharging of patients has been deferred.
Special "control measures" have been put in place, and the hospital will continue to monitor the situation daily. Elective surgery, which was postponed yesterday, will again be suspended today.
The first outbreak of the virus this year occurred last week, and the NWHB has reiterated its earlier call for the public to co-operate with new visiting arrangements.
"Although public visiting hours will continue as normal, the hospital has advised that any member of the public who has had symptoms of vomiting or diarrhoea, or has had these symptoms in the past three days, to refrain from visiting the hospital or attending for appointments," a spokeswoman said.
"Anyone who has been in contact with a person suffering from these symptoms - especially a family member - should also refrain from visiting or attending the hospital for appointments."
The spokeswoman added that the situation would be reviewed on a daily basis.
Last year both general hospitals in the NWHB region - in Letterkenny and Sligo - were hit by outbreaks of the virus.