Met Éireann has issued a fog warning for Connacht, Cavan, Monaghan, Donegal, Dublin, Kildare, Longford, Louth, Offaly, Westmeath and Meath.
A Status Orange weather warning has been issued by Met Éireann.
AA Roadwatch has said that fog is lingering in many areas, warning motorists to take particular care in the midlands and south.
Motorists are advised to use fog lights in affected areas but ensure to switch them off in normal driving conditions.
Heavy fog in Co Mayo has led to significant disruption at Ireland West Airport Knock.
Two outbound flights have been cancelled so far today and six flights into the airport have had to be diverted to Derry, Dublin, and Belfast.
Management at the airport are hoping intending passengers on scheduled services to Liverpool, Stansted and Luton airports in England - and another flight to Lanzarote - can be accommodated later today.
The situation is under constant review but foggy conditions have persisted in the area throughout the day.
This morning motorists experienced poor driving conditions and greatly reduced visibility due to dense fog in many areas.
The fog cleared slowly during the day but it is now expected to persist this evening and early tonight.
Foggy day in Dublin. #weather pic.twitter.com/3Gzi4MwaWS
— Micheal Mac Suibhne (@MicMacSuibh) November 20, 2014
Nights like this you miss the foghorns pic.twitter.com/FYCdPa7BKx
— Colman O'Sullivan (@colmanos) November 20, 2014
