Sligo General Hospital is restricting visitors in an attempt to stop the spread of the winter vomiting bug.
The Health Service Executive said more patients than usual were presenting with vomiting and diarrhoea, and this led to increased pressure on beds.
The HSE said visiting was permitted only in exceptional circumstances by immediate relatives and only during designated visiting times.
The vomiting bug, while rarely causing severe problems, can be debilitating to small children or older people who are already ill or infirm.
Those affected should drink plenty of fluids, maintain strict hygiene and avoid visiting hospitals or nursing homes.
If symptoms persist, patients are advised to contact their GP by phone before attending the surgery to limit the spread of the virus.