Last night was officially the coldest since the start of winter, with temperatures falling as low as -8 degrees.
Motorists were warned to take extra care on the roads, with bright, but frosty mornings expected for much of this week.
“At one of our road ice stations in Monaghan, we got [a reading of] -8 degrees,” said meteorologist Siobhan Ryan of Met Éireann.
The weather service measures temperatures at about 50 road ice stations throughout the country.
“In the midlands we were getting temperatures of -6. Dublin and the east coast fared a little better with -2 and -3 temperatures there. The same applies to the southern fringes. There were slight winds with a little easterly breeze,” said Ms Ryan.
In the past week, temperatures in general were between -1 and -3 degrees below average, with the coldest of those across Ulster and north Leinster.
“We had [average temperatures] of -3.5 in the midlands, south Leinster and -2 stretching from south connacht down to Waterford. Elsewhere it was -1 to -1.5,” Ms Ryan said.
Met Éireann is forecasting similar, frosty conditions tonight.
“We had -8 across Monaghan last night and it probably won’t get as cold there tonight because there will be some cloud pushing down across the north east.
“There will probably be some patchy rain or drizzle, mainly across the north eastern fringes that might move further inland. There is a possibility of some sleet on hills,” said Ms Ryan.
Elsewhere it will be dry with clearer skies and the lowest temperatures likely across the south midlands, with averages of -6 in that region.
Apart from some drizzle across northern coasts, it will be dry and bright after a cold frosty start again tomorrow.
Ms Ryan predicted “crisp” weather for the rest of the week.
“The week ahead looks like remaining much the same, becoming a little less cold as we progress towards the weekend, but ever so slightly. Later on, the weekend will become windier and a little bit milder, but with rain too,” Ms Ryan added.
AA Roadwatch said most of the road surfaces on national routes had been treated and that motorists should use them where possible.
Drivers should also reduce speed significantly and keep a safe distance from the vehicle in front.
An Garda Siochána advised drivers to avoid the Sally Gap in Wicklow due to icy conditions, as a number of vehicles got stuck there yesterday.
There are also icy conditions today in Donegal, Leitrim, Cavan, Laois and Cork, AA Roadwatch said.
Roadworks begin today on the Dundalk to Carrickmacross road (R178) until early next year and diversions are in place. HGVs must take diversions via Castleblayney or Ardee.