The HSE's Health Protection Surveillance Centre is urging people in at-risk groups, like the elderly and pregnant women, to get vaccinated.
The number of reported cases of flu-like illnesses is now above the threshold levels which means the disease is actively circulating in the community.
So far its been identified mainly in residential facilities for the elderly and disabled.
The HSE has said the flu outbreak is expected to increase over the coming weeks and circulate for up to two months.