The cold snap will be with us right through the weekend and into next week, Met Éireann has said.
The forecaster has extended a Status Yellow warning for low temperatures and ice into Sunday.
It will stay very cold with widespread frost and icy conditions. There will also be some freezing fog.
There is a warning of hazardous conditions on roads and paths, as well as some travel disruption and animal welfare issues.
A Status Yellow warning for fog has also been issued from Friday evening, warning of "dense fog forming in many areas, lingering through the day in some places."
Met Éireann meteorologist Aoife Kealy told Newstalk Breakfast it will be bright but chilly.
"It is going to be a very cold but quite bright day, some scattered showers around - mainly in the north and west," she said.
"Some of those showers will fall as hail or sleet; and the highs today will really only get up to between 2C and 6C.
"We do still have a Yellow Low Temperature/Ice warning out nationwide for tonight.
"You're going to see some showers moving in overnight, possibly of sleet or snow.
"With the temperatures dropping down to between 0 and -4 there will be a widespread sharp frost and icy patches too.
"To make matters worse, then it is going to turn quite foggy in parts overnight with freezing fog.
"All that's going to leave rather hazardous conditions tomorrow morning with that thaw, ice and fog lingering in places."
Ms Kealy said people should take extra care if they're out and about tomorrow.
"It is going to be quite dry through the day, some scattered showers moving in from the west tomorrow," she said.
"Once again, because it's so cold, some of those showers are going to turn to hail, sleet and maybe even snow.
"It does look like this colder weather is going to stay with us through the weekend, and even into the early days of next week," she added.
The weather alert is in place for the entire country from 5pm this evening until midday on Saturday.