Early-morning commuters have been warned to slow down and be vigilant amid reports of black ice and treacherous road conditions.
The Road Safety Authority (RSA) said that despite a slight thaw clearing some roads of snow over the weekend, drivers should not pick up speed as some roads are still lethal.
Noel Brett, RSA chief, said the current conditions are the most dangerous of the big freeze so far as people can be caught off guard.
"A thaw may have cleared snow from some roads, in fact they may look safe to drive normally, but they are still lethal," Mr Brett said.
"So don't get lulled into a false sense of security, we are still experiencing freezing temperatures and roads remain icy and extremely dangerous."
Although temperatures crept up to 5C in the south west during Sunday, Met Eireann warned of freezing fog becoming widespread overnight and for a time on Monday.
The respite from the wintry showers was shortlived with forecasters predicting further outbreaks of snow spreading south-west at the start of the new week.
Temperatures could plummet as low as minus 9C overnight on Sunday in some parts, with the wintry showers pushing into the north west by Monday morning.
The cold weather claimed the life of an 85-year-old man who was found dead near his home outside Ballina in Co Mayo on Saturday.
Gardai believe he died after he slipped and fell on ice. He is the second pensioner believed to have died after falling in the wintry conditions.