Older people, pregnant women and other high-risk groups have been urged to get the flu vaccine following a rise in the number of cases in the community.
The HSE Health Protection Surveillance Centre said the number of reported cases of influenza last week passed the threshold used to assess winter influenza activity.
The biggest rise in flu activity was among children aged between five and 14 years. No increase occurred among over-65s.
"Influenza is now circulating in the community, it is important that people in high-risk categories get vaccinated against influenza," said HPSC director Dr Darina O'Flanagan.
Other high-risk categories include people with chronic illnesses such as lung disease, heart disease and diabetes, those with lower immunity due to disease or treatment, and residents of nursing homes. Healthcare workers and carers have also been advised to get vaccinated.
The symptoms of flu usually develop over a matter of a few hours and include a high temperature, sore muscles, dry cough, headache and sore throat. "This is different from the common cold, which tends to come on more gradually and usually includes a runny nose and a normal temperature," said Dr O'Flanagan.
"Anyone who gets flu should stay at home, rest, drink plenty of fluids and use over-the-counter remedies like paracetamol to ease symptoms. People in high-risk categories should contact their GP if they develop influenza symptoms."
So far this winter, influenza B has been the dominant virus, but influenza A is also circulating.
"Covering your nose and mouth with a tissue when you cough and sneeze, disposing of the tissue as soon as possible and washing your hands with soap and water as soon as you can are important measures in helping prevent the spread of flu," added Dr O'Flanagan.
