Efforts to combat disruption expected amid severe weather are well under way as Ireland braces itself for the coldest spell in almost a decade.
Overnight temperatures are forecast to plunge below freezing across the country as Arctic conditions, dubbed the "Beast from the East", take hold.
Motorists are being warned to be careful, with widespread frost and icy conditions expected.
Gritters are out in force salting the road networks, as transport officials implement plans to minimise disruption.
People have been urged to check in on elderly or vulnerable neighbours, while farmers have been advised to move their animals to shelter and ensure they have adequate supplies of food and water.
A snow-ice alert has been issued for Dublin, Carlow, Kildare, Laois, Louth, Wicklow and Meath from Tuesday, with up to 6cm of snow expected to accumulate by Wednesday morning.
Met Eireann warned of widespread frost and icy conditions as it shifted its yellow status warning to orange, the next level.
Northern Ireland is the subject of a yellow warning from the Met Office from the early hours of Wednesday, with heavy snow showers predicted and the possibility of power cuts.
Some parts are set to feel colder than the Arctic Circle as freezing temperatures continue to the end of the week and the mercury drops five to 10 degrees lower than normal for this time of year.
Forecasters have also warned about the compounding problem of wind chill.
As well as the warnings about snowfall, there is a low temperature warning, with lows of minus five degrees expected on Monday night.
It is expected to be the coldest snap since the big freeze of 2010.
Severe weather plans are in place for public transport operators, to help them face the snowy and icy conditions, a meeting of the National Emergency Co-ordination Group in Dublin heard.
Officials from the Department for Infrastructure in Northern Ireland warned that even with the deployment of hundreds of staff to spread tonnes of salt, drivers should be prepared for difficult conditions.
Director of network services John Irvine said: "Despite our best efforts, it is important to remember that we cannot guarantee ice-free roads even after salting.
"We all need to be mindful of the changing conditions and adjust the way we drive, ride or cycle to ensure it is appropriate for the conditions."
Into Thursday potentially gale force winds, mixed with heavy snowfall, will ensure disruptive weather for many parts.
A weather system from the Portuguese Islands and known as Storm Emma is expected to bring blizzard conditions as it hits the cold air brought down by the Beast from the East.
Homeless charities have warned of the risk to life for rough sleepers while hostel beds in Dublin are to stay open 24 hours a day to help keep people off the streets.
Focus Ireland said it is working with all homeless organisations in the city to get beds for everyone on the streets.
The Simon Community appealed to people who see rough sleepers in Belfast to contact the charity.
The threat of heavy snowfalls and subsequent frozen roads is a result of winds bringing freezing temperatures from Russia and the North Pole and precipitation from air moving north from the Bay of Biscay.
The Department of Education in the Republic said decisions on whether schools will close are expected to be taken on an individual basis as the week progresses and on advice from the National Emergency Co-ordination Group.
In the UK, Met Office forecasts said that by the end of Wednesday more than 20cm of snow may have accumulated in some parts of eastern England, Scotland and Northern Ireland.