Three people have died from the flu so far this winter season, as the number of influenza cases continues to increase.
A total of 324 people have been hospitalised, according to figures from the Health Protection Surveillance Centre.
The official monitoring threshold was crossed last week as GPs across the country reported a sharp increase in flu cases.
Six influenza outbreaks were reported to health officials last week.
Flu sets in 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 explained: "Stay out of school or work while they're symptomatic, and until their symptoms have completely resolved - and that usually takes between five and seven days.
"People who develop the symptoms of flu as well... it's important that they would cough or sneeze into a tissue, dispose of the tissue immediately, and wash their hands."
He added: "In general the flu can be managed at home: someone should rest, get plenty of fluids, and use over-the-counter medicine like paracetamol to control the symptoms.
"People in at-risk groups or people not in those groups but whose influenza is severe or getting worse should phone their GP for advice."
