Met Éireann has warned of "hazardous conditions" as an ice warning takes effect for the entire country.
The national forecaster has issued a status yellow weather warning, as temperatures are expected to drop to as low as -4C overnight.
The alert read: "Icy stretches, with freezing fog in places, leading to hazardous travelling conditions."
The warning, which was issued on Thursday morning, came into effect at 4pm this afternoon, and will remain valid until 12pm Friday, January 20.
It will be very cold tonight with frost and icy stretches, the national forecaster said.
Thursday night, it is expected to be mostly dry with some clear spells, however mist will thicken into dense freezing fog in parts with impaired visibility.
"Freezing fog" occurs in calm and dry winter conditions, when fog forms in temperatures that are below freezing.
The tiny water droplets in the air remain as liquid. They become supercooled water droplets, remaining liquid even though they are below freezing temperature. This occurs because liquid needs a surface to freeze upon.
Later in the night, some drizzle will move into west and southwest counties. Lowest temperatures of -4 to -2 degrees, colder locally across Ulster, in just light southeast breezes.
A Status Yellow Small Craft warning from Malin Head to Fair Head to Strangford Lough is also in place from 12pm this Thursday, until 11pm tonight.
"Northwest winds will reach force 6 at times," Met Éireann has warned.
Tomorrow, Friday, is expected to bring some bright or sunny spells. It will be mostly cloudy with fog possibly persisting in parts throughout the day in light southeast breezes.
"Less cold than recent days with highest temperatures of 3 to 8 degrees generally though remaining colder where any fog lingers," Met Éireann added.
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