This dramatic change is due to a switch of wind directions, forecasters said.
The past weekend felt bitterly cold with sub-zero temperatures at night and snow on high ground around Dublin because of north-westerly winds which brought chilly gusts of wind from the Arctic and Greenland.
However, in coming days, the country will be swept by far warmer south westerly winds that will send temperatures rocketing to 10C to 11C.
"The last seven days have been very cold with temperatures 2C to 3C below average for this time of the year but the week ahead will prove relatively mild with temperatures 2C to 3C above normal for mid to late December," meteorologist Siobhan Ryan told the Herald.
"Today will be cloudy but the weather will improve in the afternoon. It will be cold again tonight with 1C to 5C, some frost and the risk of ice. By tomorrow, the weather will be mostly dry with some patchy rain around the evening time, the highest temperatures of the next couple of days will be around 9C.
"On Wednesday and Thursday, they will jump to 10C to 11C.
"On Friday, there should be rain in the morning which will clear in the afternoon but will keep temperatures a bit cooler on Saturday and Sunday.
"Christmas Day, we're looking at a rather windy day but it should be relatively mild with the chance of rain later.
"Over the past week the wind was north-westerly and it was coming down from the Arctic through to Greenland to us but now we will go back to south-westerly wind coming from the mid-Atlantic with milder sea temperatures."
The warm air will push temperatures above seasonal average for the rest of the month.