Temperatures plummeted to almost -7C overnight, as the cold snap takes hold.
The lowest temperatures recorded so far was -6.5C in Mullingar, Met Eireann confirmed to Independent.ie this morning.
Motorists have been advised to reduce their speed and leave extra braking distance as road conditions in many parts are very slippery.
AA roadwatch said this morning that road temperatures have fallen to below -7C in some places.
Monaghan and Cavan are particularly poor, as parts of Waterford and Kilkenny are gripped in thick fog.
Motorists are reminded that braking distance is ten times longer in icy conditions.
A yellow weather warning - issued by Met Eireann as the freezing conditions hit - is expected to remain in place into this morning.
But the country is set to enjoy a mild end to 2014, with sub-zero temperatures being replaced by highs of up to 12C on New Year's Day.
Following several days of very cold weather, the outlook for the remainder of the year is more favourable.
"The cold spell should disappear quickly," Met Eireann's Pat Clarke told the Irish Independent.
Day temperatures on New Year's Eve will be around 9-11C. "New Year's Day won't be too dissimilar, with temperatures of about 9-12C which is much milder than normal," Mr Clarke added.
"The past week has reflected all of December - with some cold weather, some wet weather, some mild weather and then frosty weather. We've had quite a mixed bag indeed."
The icy conditions will trail off from today, but the Road Safety Authority (RSA) is still urging drivers to be extremely careful over the Christmas period.
"The roads are incredibly busy right now, with local events like hunts and runs and sporting events and just generally more people on the move.
"The key message is to be aware that the roads are busier, and as drivers we really do need to expect the unexpected and slow down," the RSA's Brian Farrell said.
People have also been reminded to keep in touch with elderly neighbours. Age Action Ireland is advising older people not to venture outside unless necessary.
"If older people are going to listen to that advice they may need a neighbour to call in on them to check for fuel, food and medications.
"We need people to be good neighbours," the group's spokesman Eamon Timmins said.