Road conditions in the Dublin area were dangerous after freezing temperatures and snow led to icy surfaces.
The cold spell tightened its grip as it emerged that the extreme weather earlier this year cost a colossal €297m in insurance payouts.
A prolonged freeze disrupted businesses and schools as travel was made hazardous. Damage was also caused to homes and other buildings all over the country.
Hard-pressed retailers feared that today's bitterly cold temperatures combined with the Irish Congress of Trade Unions (ICTU) march would deter many families from beginning their Christmas shopping in the city centre.
As retailers and restaurants head into the vital Christmas period, shoppers were expected to stay away from Dublin city centre in their droves.
Met Eireann said the areas worst affected by the overnight snow were eastern parts of Leinster, Donegal and Connacht.
The bad weather is expected to last well into next week, with depths of up to 10cm possible in places.
A sharp ground frost is forecast for tonight, leading to extremely slippery roads and paths.
A thunder and lightning storm last night led to disruptions in transport services today, with trains in Dublin worst hit.
The DART and the northern commuter and Maynooth commuter lines were not running this morning following lightening damage overnight.
Belfast and Rosslare train services out of Dublin were also affected.
Flights out of Dublin Airport were disrupted following the closure of the main runway from early morning due to snow and ice.
Even after the runway opened again, delays were experienced.
Met Eireann, which has a weather warning in place, forecast further wintry showers tonight in counties in the east, north and north west.
Severe weather warnings were also in place right across the UK, with heavy snow, widespread icy roads and bitter temperatures forecast.
Overnight temperatures across the UK plunged well below zero again.
Scotland and the north east of England have suffered the worst of the bad weather, with snowfalls of up to 30cm recorded in the Highlands and North Yorkshire.
New figures show the big freeze late last year and in early 2010 led to the biggest ever insurance bill for a single weather event in Ireland.
The Irish Insurance Federation revealed there were 22,450 claims from the public, the vast majority of which involved damage to homes.
But the €297m could be surpassed this winter, given that the cold snap has already started.
The extreme weather last winter only began in late December.
Meanwhile, protesters took to the streets of Dublin today to protest against the country's austerity budget.
The ICTU-organised march saw thousands of people demonstrate against the four-year National Recovery Plan.
They were also protesting against the Government's planned austerity Budget, which will be published on December 7.
Last night's thunder and lightning storm came before snow fell.
However, as the morning progressed, there were more flashes across the sky.
As well as the weather warning, Met Eireann had a gale and small craft warning in effect.
Forecasters say that, from tomorrow until Wednesday night, "Ireland will remain in an exceptionally cold and sometimes strong, northeast to north wind flow".
There will be frequent wintry showers throughout the period, many of which will fall as snow.
This will result in accumulations of snow on both low and high ground.