Road users are being warned to drive with caution tonight and tomorrow morning as temperatures could go as low as -6.
Roads in shaded areas did not rise above freezing.
Black ice has been reported this evening in parts of West Wicklow.
Traffic is set to be extremely heavy on all main routes tomorrow morning as people return to work after the Christmas holiday and the schools reopen.
Turning very cold tonight with lowest temperatures falling between -4 and -6 degrees away from southern and eastern coasts.
Tomorrow will be dry in most places. Frost will clear, and most places will have a bright start with sunshine. However cloud will continue to increase from the south and west through the course of the day and a few spots of rain and drizzle may appear in the south in the afternoon possibly turning to sleet for awhile in places. Winds light to moderate southeast. Becoming milder with highest temperatures 4 to 6 degrees