Up to four people have died from the latest outbreak of the H1N1 flu virus, also known as swine flu.
The HSE has confirmed it has identified the strain in a number of patients and warned it expects more people will be affected in the coming weeks.
Signs of infection vary in severity and include symptoms ranging from aches and pains and a chesty cough to sneezing or a loss of appetite.
However, in more severe cases symptoms can escalate, leading to more serious conditions such as pneumonia.
Dr Kevin Kelleher, the HSE's assistant national director for public health, said the strain had been identified in up to four people who had died and is likely to be found in more patients in the coming weeks.
"There are two types when we talk about this virus - A and B. This year we have the A strain, which has a particular predilection for some people under 65," Dr Kelleher told the Irish Independent.
"It has a predilection for people who have a previous medical condition like asthma or respiratory disease and for pregnant women. What we see is people with diseases end up in ICU [intensive care unit] or on a ventilator."
Dr Kelleher confirmed at least two people have died from the virus, with two more suspected deaths as a result of the flu.
The HSE uses a scale to determine when the flu season fully begins which incorporates a number of elements.
The most recent indication is that the scale has reached over halfway to surpassing the flu season threshold.
This has sparked concerns the official season, where hundreds become sick with the virus, is only days away.
"I can see in the data I have available that it is beginning to rise, that it almost certainly will go above a threshold that we have got that says it is flu season," Dr Kelleher said.
"The number we use is 17 to indicate that threshold and we are at 11 now.
"The flu seasons last anywhere from 12 weeks up to 18 weeks. Last year it was 17 weeks, which was really very long."
Dr Kelleher said hundreds die from the virus every year and has made a plea for members of the public to get the flu jab and limit their contact with others if they notice any of the symptoms.
Mr Kelleher also said any children with symptoms should not attend school.
He said the vaccine was a "good match for circulating strains".
A spokeswoman for Health Minister Simon Harris said that it was not too late to get vaccinated and urged the public to check the HSE website undertheweather.ie.
The website has tips and advice for anyone who wants to prepare or prevent themselves from catching flu.