Met Eireann has this afternoon issued another Status Orange weather alert, warning that the fog may become more extensive and dense later this evening.
"Fog will persist in some parts of Northern Ireland during Thursday afternoon, thickening up towards dusk with the visibility falling below 100 metres in places".
He said: "It happens when you get clear skies and it cools down and you get fog patches forming out of these skies".
One forecaster warned that a "potent wintry blast" is on its way and could arrive in the United Kingdom "early next week".
The has issued an urgent yellow warning and said some flights could be delayed because of patchy fog on Friday night.
Light winds will stop mists clearing leaving many parts of the country under a thick pea-soup through much of the day.
The gardai also warned motorists to take care this morning.
A Met Office severe weather warning covers a portion of western Britain through tonight until 11am tomorrow.
'We have already seen fog around and it will continue over the next couple of hours and into today.
Some roads and pavements may become icy, increasing the chance of injuries from slips and falls.
The second highest level of warning advises that there will be very poor visibility in "many inland areas" this afternoon and evening and it is expected to worsen later this evening and tonight.
"Slower journey times are possible and there is a chance of delays to flights".