A Status Orange weather warning was issued for the whole country last night, applying from this morning until midnight tonight.
"Dense fog may linger in some parts of the country through much of Monday giving rise to poor visibility and hazardous driving conditions," said Met Eireann.
Munster, Connaught and the midlands are the worst affected so far, according to AA Roadwatch.
Status Orange Fog Warning in place for country today valid until midnight. Munster, Connacht & midlands worst affected so far. Use fog lights when needed; switch them off when no longer necessary. pic.twitter.com/W7OiuNbkkK
— AA Roadwatch (@aaroadwatch) December 18, 2017
The fog will be especially dense close to rivers and lakes.
Other than that, today will be mainly dry and sunny with light winds.
Later it will become cloudy and dull with some drizzle in the west and southwest.
Temperatures will be between 5 and 7 degrees generally, but higher along the west coast and lower in areas affected by dense fog.