Nearly 900 people are in hospitals across the country being treated for flu, 300 more than just a week ago.
The increase has put GPs under pressure and there is causing worry that hospital emergency departments will come under greater pressure too in the coming days.
Health Service Executive Chief Clinical Officer Dr Colm Henry has encouraged people to consider alternatives to hospital if they become unwell.
"Self-care for those for whom the 'flu does not represent a serious threat to their health, the use of local injury units, we have 14 of these across the state open from 8-8, and no appointment is necessary and they can deal with minor injuries cuts, minor burns and so on and even some fractures.
"And of course, our emergency departments are open for 24/7 for those who do need urgent, emergency time-critical and life-threatening care."
Speaking on RTÉ's Morning Ireland, Dr Henry said "sustained pressure right across the whole healthcare system throughout January is expected."
However, he said capacity is there but added that some hospitals were under strain and the steady discharge of patients needed to be maintained.
"We have extra diagnostics in place. We have senior decision-makers at the front of the hospital and up in the wards ensuring timely access to the right opinions to make sure people get out of the hospital once their stay is no longer necessary."
Meanwhile, Cork GP Dr Diarmuid Quinlan said general practices were very busy, as expected, but added that there was an uptick in the number of patients seen in GP out-of-hours on Christmas Day and St Stephen's Day compared with last year.
"The number of patients seen in GP out of hours on Christmas day and Stephen's day is up 10% compared to the year before, however, we are working with our partners in HSE and the Irish medical organisation."
Other than during the upcoming public holiday, he said most GP practices are open, adding most practices, had surge capacity and deferred routine care to make appointments available for patients who need to be seen quickly.
"So, we’ve put a lot of mitigation in place to support the expansion in flu... but we do know that we will reach a peak, but also that the flu will go on for a long time well into January."
Also speaking on Morning Ireland, he encouraged people to go out and get the flu vaccine.
"It protects yourself, protects your family, protects your community".
Dr Quinlan said that if people were unwell and suffering from symptoms of respiratory illnesses - "cough, colds, fever" - they should stay at home and not socialise and spread their illness.
