The Health Service Executive has said there has been an increase in the number of reported influenza-like illnesses in the past week.
People in at-risk groups are being urged to get vaccinated against the flu.
These include anyone aged 65 and older and all pregnant women.
The rate of reported cases has risen from 20.5 per 100,000 of the population to 32.4 per 100,000 in the past week.
HSE's Health Protection Surveillance Centre specialist in public health medicine Dr Joan O'Donnell said it is not too late for people who are at risk of the complications of flu to get vaccinated if they have not already done so.
Symptoms usually develop over a few hours and include a high temperature, sore muscles, dry cough, headache and sore throat.
Dr O'Donnell said anyone who develops flu symptoms should stay at home and rest, drink plenty of fluids and use over-the-counter remedies such as paracetamol to ease symptoms.
Anyone in high-risk groups should contact their GP if they develop flu symptoms.