Met Éireann issued a national weather warning, telling people to "be aware" as wintry showers take hold for much of today, falling as sleet and snow in many areas.
The status yellow warning is also in place for small aircraft and on the seas, as westerly winds increase to gale force along exposed coasts, bringing little respite for fishermen.
Today starts very cold and windy, with widespread morning showers turning wintry, especially in the north and west, with some snow.
Temperatures will only reach 2C to 5C, with strong and blustery winds making it feel all the colder.
It will remain wet and windy tonight, with wintry showers becoming widespread - and temperatures dropping to well below freezing, with hazardous driving conditions.
The Road Safety Authority last night issued an alert, warning of snow and black ice over the coming days.
The worst of these will be in parts of Connacht and Ulster, where showers of snow are expected overnight, with accumulations of several centimetres in many areas, especially on higher ground.
And it looks as if these severe conditions won't ease on Thursday, when snow is likely again. Snow showers will be frequent over hills and mountains in the west and north, though most areas are unlikely to escape unscathed.
Low temperatures of only 3C to 4C won't be helped by the wind chill factor. The temperature will drop further tomorrow night, to below freezing again, though the showers will finally start to retreat to western and northern coasts.
One sector that's really feeling the pinch is the fishing industry, particularly the smaller boats that have been unable to get out since before Christmas.
Dingle-based fisherman Michael Hennessy said the weather had been too broken for the smaller boats to venture out and it was looking like this would be the same for another week.
"It's supposed to get finer again in another week and hopefully, we'll be able to make some start then," said Mr Hennessy, skipper of the Reált na Mara.
But for the time being, wintry conditions will prevail.
By the week's end, showers will start to retreat to coastal areas. By Friday, some sunny spells are expected, particularly in the east and south, although it will remain cold.
But then the airflow will turn northerly on Saturday, with wintry showers continuing, mainly in northern and western areas. Good periods of weekend sunshine will not prevent severe frosts and icy roads by night.