The HSE is asking people to stay home from work or school if they are displaying symptoms of flu.
The official monitoring threshold was crossed last week as GPs across the country reported a sharp increase in flu cases.
Flu sets far more quickly than a cold and symptoms include a high temperature, body aches, a dry cough and a headache.
Dr John Cuddihy said avoiding contact with people is the best way to stop the illness spreading.
He sadi: "Stay out of school or work or creche while they're symptomatic until their symptoms have completely resolved, and that usually takes between five and seven days.
"People who develop the symptoms of flu, it's important that would cough or sneeze into a tissue, dispose of the tissue immediately in the bin and wash their hands."