The HSE has set up a new crisis team to deal with a winter virus surge, as it expects to face the "highest pressure" ever in the coming weeks.
In a statement, the HSE said flu and Covid-19 infection and hospitalisation numbers are likely to rise "very rapidly".
More than 900 patients could be in hospital with flu in the first week of January with that number continuing to rise further, according to HSE analysis.
The health service is also seeing a higher number of hospitalised Covid-19 cases this winter than had been anticipated in pessimistic projections.
As of 8am on Thursday, there are 656 people in hospital with Covid-19, 26 of whom are confirmed cases in intensive care units. There are about 1,200 people currently in hospital with respiratory conditions.
The HSE said the surge in respiratory illnesses will seriously impact hospitals and Emergency Departments, and will place primary care services such as GP and GP out-of-hours services under further pressure.
In response, it has established a National Crisis Management Team (NCMT), which will give national support and guidance to the response of hospitals and community services throughout the country.
HSE chief Stephen Mulvany said it was looking "increasingly likely" that demand for health services will rise well above anything seen before in Ireland.
He said: "It is therefore critical that we continue to prepare to the maximum level possible for the projected further surge in demand for services in late December and early January – over and above the current high level of demand.
"We are working to ensure that every available resource is mobilised and utilised to respond to the needs of our patients who are seeking urgent and emergency care.
"We would also urge all those eligible for and due to be vaccinated to do so as soon as possible."
