Dublin's Beaumont Hospital has extended its ban on all visitors for the weekend.
The restrictions are part of an effort to prevent the spread of the winter vomiting bug.
The hospital is urging anyone suffering from nausea, vomiting or diarrhoea to seek advice from their GP before visiting family or friends.
"We're asking [people] not to come in unless it's absolutely essential," said Hillary Humphries, a consultant at Beaumont.
"In that case, whoever's coming in, that person should contact the ward staff and register, so we know who it is that's going to come in, so it's an individual who's been flagged in advance, who has got permission to come in and see their relative in hospital."