The Chief Operations Officer with the HSE has warned the flu virus is 'still to hit its peak' amid climbing numbers of infections.
Damien McCallion was speaking as the HSE issued an appeal to people seeking medical care or assessment to consider all options before going to a hospital Emergency Department.
It expects it to be one of the busiest ever periods experienced by the health service.
The appeal comes as a new record has been set for overcrowding in Irish hospitals, according to the latest figures from the Irish Nurses and Midwives Organisation (INMO).
931 people are being treated in trolleys or chairs across the country.
Some 767 are in Emergency Departments, 164 are elsewhere within the facility, while 26 children have been admitted to hospital without a bed.
University Hospital Limerick is the most overcrowded, with 97 patients on trolleys, followed by Cork University Hospital where 74 patients are waiting.
All outpatient appointments for University Hospital Limerick have been cancelled with only urgent elective surgeries going ahead at the hospital "over the coming days."
The Mater Hospital in Dublin is also urging people with non-urgent conditions to avoid coming to its Emergency Department.
Mr McCallion told Newstalk Breakfast there are several factors to this.
"What we're experiencing at the moment are particularly high levels of respiratory illness in terms of COVID, flu and RSV," he said.
"We've never had that before; we've over 700 patients in hospital with COVID, and 700 patients with flu.
"Those numbers are continuing to rise."
He said they are currently in the 'escalation' part of the Winter Plan.
"These are things like using the private hospitals, additional sessions being run by GPs, and also then in terms of prioritising the urgent and emergency work," he said.
"We knew this would be unprecedented this year and every effort was made to plan for that.
"We have to work through these couple of weeks.
"We know this is going to be a difficult period, as the flu is still to hit its peak, and that's the reason we're making that plea to people".
What to do
Mr McCallion said people's first port of call should be undertheweather.ie if they have flu symptoms - this could be followed by a pharmacy or GP visit, or a Local Injury Unit appointment.
"Lastly if someone has an emergency requirement, they should of course go to an Emergency Department or call an ambulance," he explained.
He said people are still being taken on for vaccinations.
"It's not too late for people still to be vaccinated," he said.
"We've had good uptake on the flu [vaccine], less so on COVID, particularly those in vulnerable groups and elderly people and healthcare workers.
"We're still encouraging people to come forward - a wide range of options [are] available," he added.
Additional reporting: Mairéad Cleary