Met Éireann has issued a severe weather warning for the next two days, with blizzard conditions expected in places today as winter tightens its grip.
Snowfall of up to 10cm has been forecast in some areas, with gale force winds, heavy rain, flooding, sleet and plunging temperatures predicted to upset travel plans.
Further falls of sleet or snow are expected today and tonight over the northern half of Leinster, Ulster and north Connacht and also on higher ground in Dublin and Wicklow. Accumulations of between 5cm and 10cm are possible, with the worst further north and on high ground.
Sub-zero temperatures will lead to icy patches while high winds in coastal areas are expected to cause some flooding.
Several flights in and out of Ireland have been cancelled, Dart services in Dublin have been disrupted, bus services in some remote areas and higher ground have been diverted or delayed while some roads are blocked as a result of the severe conditions.
AA Roadwatch said it had been a "difficult" day for motorists.
'It's been an extremely difficult day on the roads with flooding and snowfall causing major problems through east Leinster, parts of the midlands and Ulster' Aoife Carragher, head of AA Roadwatch said.
"Co Wicklow has been most severely affected with snow on higher ground, many roads blocked due to flooding and landslides around Delgany," she said.
She warned about road conditions tonight. Falling temperatures tonight and wet roads in Leinster "likely to result in a hard frost and icy surfaces with a possibility of black ice".
The weekend would bring "little reprieve" with "further showery outbreaks of rain, sleet or snow," she said.
She urged drivers to remember that gentle manoeuvres are the key to safe driving and stopping distances are 10 times longer in ice and snow.
"Major routes are more likely to have been gritted than secondary roads," she said. She advised any motorists skidding on ice not to brake fast, to reduce acceleration and reduce gears to come to a halt.
Dart services south of Dún Laoghaire to Dalkey have been cancelled due to flooding at Sandycove station and this disruption will continue for the rest of the evening, Irish Rail said in a tweet.
The Department of Transport has urged those on the move to check its website for updates. The Road Safety Authority has appealed to road users to remain extra vigilant on the roads as sleet and snow continue to fall.
Some Bus Éireann services were delayed in Wicklow and Cavan while officials were continuing to monitor routes and local conditions.
"We are coping quite well with the conditions for now," said company spokesman Andrew McLindon.
Dublin Bus said drivers were experiencing some difficulties on routes near the Dublin Mountains and north Wicklow. Diversions were put in place where there is flooding and icy roads.
Commuters in the Wicklow area were particularly hit by the weather with some motorists on the N11 northbound experiencing delays of over an hour this evening.
Snow caused disruption on roads near Kells, Co Meath. There were reports of snow in Monasterboice, Co Louth. Driving conditions were treacherous in Belfast, particularly on the Crumlin Road and Upper Hightown Road.
Aer Lingus was forced to cancel at least 16 flights between Ireland and the UK as heavy snowfall hit airports in London, Bristol, Cardiff and Bournemouth. Ryanair has also cancelled a number of services. Air passengers planning to travel over the coming days have been urged to check their flights are going ahead before leaving home.
With gale force winds, rain, sleet and hail expected over the weekend, Irish Rail said it was monitoring parts of its network which are at risk from flooding.