Snow ploughs have been used to clear roads The Roads Service NI network manager has said the recent cold spell is “the worst prolonged conditions I have experienced” in 25 years.
Temperatures in NI dipped to the lowest on record with -18C recorded at Castlederg, Co Tyrone, on Sunday night.
The Met Office has said the minimum temperature on Monday night is likely to be -15C.
The Roads Service’s Colin Brown said they have been facing “an enormous challenge” to keep main roads open.
“Forecasts are indicating the sub zero temperatures are set to continue until Christmas and we would appeal to drivers over the holiday period to slow down and drive carefully,” Mr Brown said.
“Staff will be continuing to work over the Christmas period to try and keep the main network open.”
Hundreds of schools were again closed in Northern Ireland on Monday after more snow overnight and more will be closed on Tuesday.
While the Met Office has said there is unlikely to be be much more snow overnight – except light showers across northern County Londonderry and northern County Antrim – “another widespread severe frost develops as well freezing fog patches becoming more extensive”.
Belfast’s two main airports are currently open but cancellations and delays are expected to continue. City of Derry Airport reopened at 1100 GMT.
More than 500 schools were recorded as having closed on Monday.
A number of train services were disrupted in Northern Ireland on Monday morning.
Ciaran Rogan of Translink said bus services were “sticking to main roads”, and some delays were expected on the Goldline service.
Ulsterbus services were suspended in a number of areas including Ballymena, Coleraine, Antrim and Downpatrick.
Belfast International Airport spokesperson, Deborah Harris, said on Monday it had been a frustrating weekend for passengers.
“Flights are taking off and landing this morning, albeit with delays,” she added.
“The Heathrow and London Stansted services this morning are cancelled because of issues in London.
“It is the busiest time of the year with people trying to get home for Christmas and people getting off for Christmas.”
George Best Belfast City Airport said on Monday morning it was operating, but was experiencing some cancellations and delays “as a knock-on effect from other airports on the UK mainland”.
City of Derry Airport was closed until 1100 GMT, but is now operating, although it said flights were subject to delay.
The three Northern Ireland airports have advised passengers to contact their airlines before travelling because of the impact of delays.
EasyJet said the vast majority of its flights to and from Belfast are operating.
“We have a small backlog of passengers returning to Northern Ireland, but are confident that these passengers will return home within the next day or two,” the company said.
SDLP leader Margaret Ritchie said the Northern Ireland Executive needed to set up an inter-departmental taskforce to review systems for dealing with extreme weather events.
However, Regional Development minister, Conor Murphy, has said he believes this is unnecessary.
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