Several bands of heavy rainfall will bring rainfall of 30 to 50mm in the counties, with higher totals likely in mountainous regions, according to Met Éireann forecasts.
This may lead to localised flooding and hazardous driving conditions.
A small craft warning is also in place for all Irish coasts.
Southeast to south winds will reach force six or higher at times on Wednesday on all Irish coasts, gradually easing from the southwest on Wednesday night.
The same warnings remain in place for Thursday as well.
Rain will clear Ulster and parts of Leinster through Thursday morning, bringing a bright and dry day for most with sunny spells and scattered showers. Relatively mild with highest temperatures of 10 to 13 degrees in moderate southwesterly winds.
Thursday night will be dry and clear at first with some mist and fog patches forming in light southeasterly breezes.
The lowest temperatures will be 4 to 6 degrees over the northern half of the country, and 6 to 9 degrees further south.
Friday, Christmas Eve, will see outbreaks of rain slowly spread northwards, reaching Ulster by the evening.
However, clearer and brighter conditions will move in to the west and south through the afternoon, with patches of light rain and drizzle lingering. The highest temperatures will be 8 to 11 degrees.
There is “some uncertainty” in the forecast for Christmas weekend, Met Éireann said.
Current indications suggest that more rain will spread across the country from the southwest on Christmas Eve night.
“However, with Rudolph’s help, Santa will have no problem making his way through the rain,” Met Éireann said.
Saturday, Christmas Day, will be generally cloudy and breezy with outbreaks of rain, heaviest in the southeast.
Ulster will remain dry for most of the day with highest temperatures of 6 to 9 degrees.
Sunday will continue to be breezy with outbreaks of rain across much of the country.
The weather will be changeable through the final week of 2021. Met Éireann said it currently looks like a dry and cold start to the week but further spells of rain and some breeze will arrive later in the week.