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.
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 increase was among children aged between five and 14. No increase occurred among over-65s.
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.
