For Christmas Day, parts of Scotland and Northern Ireland are likely to see some heavy and prolonged rain and there is still a possibility of a white Christmas for some northern areas of Scotland.
Despite a mild lead-up to Christmas, the forecaster reckons conditions will be unsettled and cold for much of this week.
"It will be a cold and frosty Christmas night".
The warning is to start from 9am on Sunday until 9am on Monday.More news: Union sues over Facebook job ads excluding older people
"But the rain should turn lighter in the northwest before dawn".
The warning was issued by Met Éireann at 4pm on Saturday, and is valid from midnight on Christmas Eve until 8pm that evening.
It's going to be a boring and windy Christmas Eve with some rain later in the evening with rain forecasted for Christmas Day and temperatures set to dip in the evening and frost expected overnight.
The national advisory warning then is set to impact both Munster and Leinster with heavy and persistent rain expected tomorrow.