Frost and fog will again make for difficult driving conditions today as winter starts to bite, especially in the north of the country.
Fog may be dense and slow to lift in places, with the north midlands and Ulster most at risk. Only Munster is likely to escape.
However, Met Éireann warns that worse weather is on the way by the end of the week when "bitterly cold" winds will sweep down from the north, perhaps bringing the first snowfalls of winter to high ground.
Fog failed to lift at all in Ulster's Bann Valley yesterday, and it could be slow to clear again today.
Frost and some fog will return again tonight. And although tomorrow should be milder, with more cloud cover than in recent days, it will be a short respite before wintry conditions arrive on Thursday.
The Met spokesman said it would be "very, very cold" between Thursday and Saturday, with northerly breezes adding a wind-chill factor. The winds could also bring sleet or snow to northern hills, although Scotland is expected to get the worst of it.
The jury is still out on predictions of an unusually cold winter, Met Éireann said.
The British meteorological office has forecast severe weather throughout northern Europe in the early new year, but the European Centre for Medium-Range Weather Forecasting - which includes Ireland among its members and is also based in Britain - predicts the opposite.
Long-term forecasting is based on meticulous measurement of temperature shifts in the waters of the north Atlantic - what scientists call the North Atlantic Oscillation.
"It's a very difficult science," said the Met Éireann spokesman.
