The public has been called on to stop the spread of the winter vomiting bug after almost 200 cases were recorded in one week.
Health chiefs urged people not to visit hospitals, to stay out of work and school, and to follow hand hygiene directions during the spike.
About 190 cases of norovirus were notified to the HSE Health Protection Surveillance Centre (HPSC) the week before last and 115 recorded last week, while hundreds more recover at home without visiting a GP.
Dr Paul McKeown, specialist in public health medicine, said there are also several reports of outbreaks affecting hospitals, nursing homes and hotels around the country.
"In a normal week, 50 cases would be regarded as a high number of notifications so we are in an upsurge period," he said.
"Norovirus is the gastrointestinal equivalent of the common cold.
"In any year at least 1% - and in high activity years up to 5% - of the population can expect to be affected by this virus, meaning that between 1,000 and 5,000 people could fall ill with norovirus per week during a busy period."
Anyone with vomiting or diarrhoea should not visit a hospital or nursing home until they have been well for 48 hours, and children should be kept out of school and creche.
The spread of norovirus at home and in the community can be stopped if people wash their hands regularly, use hot water and detergent to clean up any vomit, and stay off work and school. "There is no specific treatment for norovirus apart from sipping plenty of clear fluids such as water or flat lemonade," Dr McKeown said.
The medic warned that the vomiting bug is highly infectious and very resilient and can cause serious disruption in hospitals and nursing homes. "It is important that ill and vulnerable patients in these settings do not become more ill than they already are," he said.