Snow is expected to fall this week as medical experts warn the flu outbreak will worsen and hit its peak over the next two to three weeks.
Met Éireann forecasters predict snow may fall from mid-week as temperatures are set to drop dramatically.
It was a wet start to the week on Monday with a mix of sunny spells and scattered showers expected through the day.
Tuesday will be cloudy and mostly dry with temperatures of 10-12 degrees.
The change in weather is expected from Wednesday as wintry showers of possible snow, hail and sleet are forecast during the night. Temperatures are set to fall to minus 1 degrees.
Forecasters said the very cold conditions are expected to continue until Saturday as as winds will draw cold air down from the Arctic.
On Thursday, Connacht and Ulster will be worst hit by the showers as they become more frequent. Snow, hail and sleet are expected to fall across the country.
Temperatures will range from 1-4 degrees during the day and widespread frost and ice is expected during the night.
The arrival of "bitterly cold" weather could impact the flu outbreak, the assistant national director of public health and children's health with the Health Service Executive (HSE) Dr Kevin Kelleher said.
Dr Kelleher said people returning to work and children going back to school could also make a difference.
“It (the flu) will hit its peak in the next two or three weeks and then it will decline,” he said.
“If it gets very cold we move inside a lot and that means we stay inside. So, you’re more in company with other people as a consequence and more likely to pass on these things.
“Actually being outside, really outside, lessens the opportunity to pass on these things. Being in a room, in an enclosed space, increases the chance of passing on these problems.”
Dr Kelleher told RTÉ's Morning Ireland most people with the flu could look after themselves and did not need to go to emergency departments.
He said information and advice to help people fight the flu and other winter bugs was available on www.undertheweather.ie.
The advice from the HSE comes as hospitals battle overcrowding and the numbers of people on trolleys in emergency departments surged over recent days, hitting a record 612 at one stage last week.
Seven people have died as a result of the flu this winter season, according to the Health Protection Surveillance Centre.
Dr Kelleher said he did not agree there was any need to delay the opening of schools during a normal season of flu.
