The HSE's Health Protection Surveillance Centre (HSPC) has urged people in at-risk groups to get the 'flu vaccine after the number of cases almost doubled in a week.
In the week ended February 12 the rate for influenza-like illness in Ireland increased to 26.2 per 100,000 population, from a rate of 15.7 per 100,000 the previous week, the HSPC said.
The HPSC said the number of cases recorded last week are now above threshold levels which means flu is now actively circulating.
Those at risk include the elderly, people with chronic lung or heart disease, diabetics, pregnant women and those with lower immunity because of disease or treatment.