Extreme cold, wind and snow will combine to make for some of the most severe weather so far this year, Met Éireann has warned.
Weather which will "have the capacity to impact significantly on the people" in Leinster, Ulster and Munster is predicted by the national forecaster.
It has issued a “status orange” warning, the second most serious warning it can give under a new alert system where yellow warnings are issued when people should “be aware”, orange when they should “be prepared” and red when they should “take action”.
Met Éireann meteorologist Gerry Murphy said today would be bitterly cold with frost lingering in places all day and with snow showers in coastal counties of Ulster, Leinster and Munster. Driving conditions could be especially hazardous tonight and tomorrow.
“The snow will sit on the ground today,” he said, “and temperatures everywhere will struggle to get above three degrees.” There will be fresh gusty northeasterly winds in the eastern coastal counties.
Tonight is set to be even colder with air temperatures falling as low as -6 degrees and ground temperatures as low as -10. Though tomorrow would be sunnier, dry and a little warmer, tomorrow night would again be extremely cold with frost and dangerous driving conditions likely overnight.
Mr Murphy said Connacht would have pleasant, sharply cold but dry and sunny conditions.
The development of the new colour-coded alerts was to bring the Irish system into line with the European Meteoalarm system.
When applied to snowfall, a status orange alert meant between 3cm and 8cm of snow was expected, he said. Yellow would apply for anticipated snowfall of less than 3cm and status red was where more than 8cm was expected.
He said temperatures would gradually rise from tomorrow.
Temperatures would reach5 to 9 degrees with light to moderate winds and scattered showers on Wednesday, while Thursday and Friday would see more rain but temperatures reaching 7 to 10 degrees.
