Latest: Met Éireann has extended a Status Yellow warning for fog until 9am tomorrow morning.
The alert came into effect yesterday afternoon and was due to expire this morning.
Low visibility will affect all counties with fog likely to linger all day.
It will become widespread and dense again this evening but will retreat to northern areas by Sunday morning.
Meteorologist Liz Walsh says slack winds and low sun mean it will not clear from many areas.
"The fog will be slow to lift, we have got very light winds over the country at the moment and that is perfect conditions for fog to linger," said Ms Walsh.
"There will be some burn off today, it will thin out a little bit as the day goes on but it looks like it will come back and become widespread and dense again later on this evening and through the night."
The Road Safety Authority advises vulnerable road-users such as pedestrians, motorcyclists and cyclists to ensure they are clearly visible to other road-users by wearing high visibility clothing and using lights.
Any frost will clear this morning, but fog will linger in some parts through the day. Otherwise, it will be largely dry with sunny spells & just the odd light shower. Showers will become a little more frequent along W&S coasts later. Highs 5 to 8°C, but lower where fog lingers. pic.twitter.com/2q0eGR6rp9— Met Éireann (@MetEireann) December 21, 2019
“Heavy fog seriously reduces your visibility and makes driving very dangerous," said Moynagh Murdock, CEO of the RSA.
"Our advice is to switch on dipped headlights and fog lamps, reduce your speed and keep a safe distance from the vehicle in front. If the fog closes in, reduce your speed further and take your time getting to your destination.
"Open your window a little so you can listen out for other road users, particularly at junctions.
"Vulnerable road users should avoid, cycling or walking in fog unless it is a necessity.”
Other tips for road users include:
Fog may form again tonight in many areas.
Later in the night, shower will spread from the west and this should clear most of the fog.
Update 7.45am: A yellow fog warning is place for the entire country this morning, on the busiest commuting weekend of the year.
The alert came into effect yesterday afternoon, leading to dangerous driving conditions.
The warning will remain in place until 10am this morning.
But meteorologist Cathal Nolan says it will be several hours before it is completely clear.
"Certainly many of us are waking up to quite dense fog this morning and we do expect that that fog will continue on for quite a number of hours," said Mr Nolan.
"The advice to motorists is to reduce your speed, put on your fog lights and drive with care as visibility is greatly reduced due to the density of the fog which is still quite bad in many parts of the country."
Any frost will clear this morning but fog may linger through the day. It will be largely dry with some sunny spells & just the odd shower. Along southern coasts showers may become more frequent later in the day. Highest temperatures of 5 to 8 degrees, but lower where fog lingers. pic.twitter.com/8Io0FZeeRJ— Met Éireann (@MetEireann) December 21, 2019