Met Eireann has issued a status orange fog warning for much of the country.
The forecaster has said it will be dense in many parts of Connacht, Leinster, Cavan, Monaghan, Donegal, Cork, Limerick, Tipperary and Waterford.
The warning is in effect until midday.
The fog is expected to clear gradually and with some bright and sunny spells developing in Connacht and north Leinster, however fog and low cloud may affect parts of the east and southeast coast during the day.
Motorists are being asked to take extra care in reduced visibility and keep a safe distance from the car in front.