Status Yellow fog warning is now in place for the entire country.
Met Éireann said the warning came into effect at 5.15am and will remain in place until 10am this morning.
Motorists are being warned that some areas of dense fog may cause difficult travel conditions.
“Areas of thick fog probably causing some tricky travel conditions or delays this morning,” the forecaster said.
The UK Met Office also issued a Yellow fog warning for Antrim, Armagh, Down, Fermanagh, Tyrone, Derry which came into effect at 4.40am, and will remain in place until 10am.