A severe weather warning has been issued by Met Éireann as overnight snowfall made driving conditions treacherous and caused the cancellation of many flights between Ireland and Britain.
Met Éireann issued a further warning for tonight and tomorrow. It said snowfall is expected to be up to eight
centimetres deep in some parts of the country.
Aer Lingus was forced to cancel all its flights to and from London Heathrow today, with further disruption expected tomorrow. Routes to Gatwick were also experiencing lengthy delays.
Ryanair flights from Shannon to Stansted and flights from Belfast City Airport were also cancelled.
Dart services were suspended between Bray and Greystones, Co Wicklow, after a rock fall close to the entrance of the first tunnel on Bray Head. Buses were arranged to transfer passengers expecting to travel on Irish Rail evening services between Rosslare, Arklow and Gorey and Dublin.The disruption is expected to last for several days.
Gardaí warned motorists to use extreme caution in some parts of the country, particularly around Naas, Co Kildare, where ice and snow made driving conditions extremely hazardous. The M7/M9 junction was said to be under a sheet of ice, while there was a warning of black ice on the N7 near Roscrea, Co Tipperary.
A general warning was also given to motorists in the wider Carlow and Kildare areas, while the Sally Gap and the Wicklow Gap, both in Wicklow, were impassable.
"Driving conditions are extremely hazardous and conditions are expected to deteriorate," said a Garda spokesman. "Motorists are asked to slow down and only make their journeys if absolutely necessary."
Adding to the misery for many commuters, flooding in Clonmel, Co Tipperary saw roads and parts of the town closed off, while minor crashes were reported on the Clonmel to Kilkenny Road because of black ice.
Schools were reported shut down in Donegal, Carlow, Meath and Kilkenny because of the treacherous conditions.
Met Éireann said snow showers affecting much of Leinster and west Ulster will spread to all other parts of Ireland tonight and tomorrow.
The snowfall is expected to be between three to eight centimetres in many places with widespread penetrating frost and temperatures plummeting to as low as -3 degrees Celsius during the night.
There was also a gale warning in place, as north-east to north winds were expected to reach gale force at times on Irish coastal waters.
The snow and sub-zero temperatures drew an appeal from Age Action for people to keep in contact with their older neighbours during the week.
"We are reminding people to call in on their older neighbours to ensure they have enough fuel and basics such as milk and bread," Age Action spokesman Eamon Timmins said. "Older people can also play their part by keeping in telephone contact with each other and with their family and neighbours."
Mr Timmins advised older people to maintain a comfortable temperature indoors, dress up warm with several layers, stay active and eat well.
In its monthly report for January, Met Éireann said it was the coldest start to the year since 2001, while it was the coldest January at Cork Airport since 1997.
Additional reporting by PA