Motorists faced treacherous road conditions today as workplaces reopened following the Christmas break.
Driving was made extremely difficult across many places this morning after temperatures plunged to -8 degrees overnight.
And the country will remain in the grip of the cold snap, with Met Eireann saying further frost and snow will hit tonight.
The icy weather is due to last for at least another week, having begun in the middle of December.
Met Eireann says the last time the country experienced such a prolonged freeze was in the early 1960s.
Secondary routes were very dangerous all over the country today and road closures remained in place.
The worst affected areas were Dublin, Kildare, Laois and other parts of the midlands.
Conditions improved on primary routes since the weekend after extensive gritting was carried out.
Dublin's Lord Mayor Emer Costello appealed to motorists not to make any unnecessary journeys.
AA Roadwatch warned motorists to proceed with particular care on back roads and in housing estates in Dublin.
"Bear in mind that most people are back to work this morning so there will be extra traffic on the roads," the AA said.
It advised extreme caution in Lucan, while gardai urged motorists to avoid the Strawberry Beds, Tinkers Hill and Knockmaroon Hill areas due to ice.
The Chapelizod, Islandbridge and Knockmaroon gates of the Phoenix Park were closed as a result of the treacherous conditions.
Ice also affected Glasnevin, Carrickbrack Road in Howth and Oscar Traynor Road near Santry.
Conditions were extremely slippery on the Dundrum bypass and on Butterfield Avenue in Rathfarnham.
Dublin Bus operated a full service today, though some curtailments were in place. Its website, www.dublinbus.ie, carried regular updates of affected routes.
Bus Eireann said the vast majority of its services were operating today, with some minor delays.
"There are some local services affected in the Meath area at the moment. Obviously we are continuing to monitor the situation very closely throughout the day," said company spokesman Andrew McLindon.
Mayor Costello said: "I urge all motorists and pedestrians to stay safe and not to make any unnecessary journeys."
She added: "I am appealing to all Dubliners to look in on those vulnerable neighbours who may need small things like groceries from the local shop, or simply just a friendly reassuring face to talk to during this difficult time."
Across the country, secondary roads were extremely dangerous this morning.
Gardai appealed to motorists to drive with caution on the M7 between the Naas North and Monasterevin junctions due to black ice.
It was slippery on the Blessington Road, coming out of Naas, while there were icy conditions around Kildare town and on the Kildare to Rathangan Road.
Overnight snowfall meant driving was difficult in Navan, Co Meath, while the N52 Kells to Carlanstown Road was also dangerous.
Thousands of tonnes of grit arrived in the country today for collection by local authorities.
The National Roads Authority (NRA) said the prolonged freezing weather has resulted in two-thirds of the season's supply of grit being used.