The first case of flu this season was detected in the past week, according to the national Health Protection Surveillance Centre.
The type identified was influenza A (H3) and was detected by the National Virus Reference Laboratory in a sample taken from a female patient.
Dr Darina O'Flanagan, director of the Health Protection Surveillance Centre, said the first case this season was detected slightly earlier than last year.
It served as a reminder to people in at-risk groups to get the flu injection even at this stage, she said.
At-risk groups include those over 65 years; people with severe illness such as chronic heart disease, chronic lung disease and diabetes; those with lower immunity due to disease or treatment including those who have had their spleens removed; children or teenagers on long-term aspirin therapy; residents of nursing homes, old people's homes and other long-stay facilities; and healthcare workers.
"The symptoms of influenza infection 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," Dr O'Flanagan said.
"Anyone in the high-risk categories who develops influenza symptoms should contact their GP," she advised.