The Health Service Executive has asked people with symptoms of the winter vomiting bug to stay away from hospitals and healthcare centres.
The body said the measure was necessary to prevent the spread of the virus, which forced the quarantine of 180 beds at Tallaght Hospital yesterday.
The HSE said the winter vomiting bug was not a serious illness in itself and people showing symptoms should stay at home from work or school, drink plenty of fluids and telephone a GP if advice is needed.