Many places experienced the coldest night of the winter last night as temperatures plummeted overnight and an exceptionally mild December gave way to a freezing January.
Temperatures were scheduled to fall to minus 6 degrees in the east midlands, making it the coldest night for almost a year.
There were also the first flurries of snow in east Ulster and along the eastern seaboard.
The cold conditions have been caused by easterly winds.
They caused heavy falls of snow in Scotland and the north of England, leading to the closure of many roads.
The easterly winds give way earlier today to southwesterly winds.
Although such winds are traditionally associated with warmer weather, temperatures will be as low as 3C in Leinster.
There is little chance of respite as the weekend is also forecast to be cold, with wintery showers and the chance of frost and icy patches.
Monday night will also be cold, with temperatures at about zero in many areas, but winds should prevent significant frost.
The long-range forecast is no better for the rest of next week.
"It is quite unsettled and cold, but not unusual for this time of year," said Met Éireann forecaster Siobhán Ryan.
"It is going to be extremely cold today and staying quite windy, with the possibility of widespread frost."
