MET ÉIREANN has said severe frost, icy conditions and freezing fog are leading to hazardous travelling conditions today, with many flights in and out of Ireland also being disrupted.
A Status Orange freezing fog warning is currently in place for most of Ireland.
The warning has been issued for Connacht, Carlow, Dublin, Kildare, Kilkenny, Laois, Longford, Louth, Meath, Offaly, Westmeath, Cavan, Donegal, Monaghan, Clare, Limerick and Tipperary and will run until midnight.
Freezing fog will be dense in places and will persist through the afternoon and into tonight, leading to very hazardous driving conditions, Met Éireann has said.
The forecaster said temperatures last night were between -1 and -5C in most places, with Athenry recording -7.2C.
Met Éireann has also issued a Status Yellow low temperature and ice warning for Ireland, valid until midday on 16 December.
It has warned of widespread sharp to severe frosts with some showers of hail, sleet and snow, mainly near coasts, and freezing fog at times.
The forecaster is warning of treacherous conditions on paths an roads, travel disruption and potential supply disruption.
Meanwhile, more than 25 inbound and outbound flights have been cancelled at Dublin and Shannon airports today.
Nine inbound flights and 12 outbound have been cancelled so far today according to the Dublin Airport website, with that number expected to rise due to weather conditions in the UK.
The departing flights include Aer Lingus and British Airways services to Leeds Bradford, Heathrow, London City and Birmingham, while the cancelled inbound services from those airlines are from Heathrow, London City, Donegal, and Birmingham.
There are also cancelled Delta Airlines services to and from the US city of Boston.
Shannon Airport is also reporting eight total cancellations.
The National Emergency Co-ordination Group met this afternoon and will meet each day this week as Met Éireann warned low temperatures will continue until Friday.
A statement from the group said that most schools remain open but a small number of schools were closed today due to hazardous local conditions.
Milder conditions were expected to return at the weekend, it said, adding that public transport is running but may be a bit slower than normal in some areas.
The UK will stay cold through this week with the risk of sleet and snow at times continuing, especially in northern and eastern coastal areas. Overnight frosts will be severe in places and we will continue to see some freezing fog patches.
Met Office Chief Meteorologist, Matthew Lehnert, said:
"The cold conditions will remain in situ during this week. In many places daytime temperatures will struggle to get above freezing, while overnight temperatures have the potential to drop below -10 C in rural parts of Scotland."