MET Éireann has issued a Status Yellow rainfall warning for Munster and Leinster with torrential showers of up to 35mm set to batter the Emerald Isle.
The weather alert kicked in at 3pm on Tuesday afternoon across all 18 counties of the two provinces and will be in place 6am on Wednesday morning.
Downpours of 25mm to 35mm of rain are expected tonight with showers heaviest towards the southeast.
Ireland's national forecaster said: "Status Yellow - Rainfall warning for Munster and Leinster.
"A spell of heavy rain will move up from the south Tuesday afternoon, giving 25 to 35 mm of rain.
"The larger totals are likely to be over southern and eastern counties."
Met Éireann previously warned that rain will be "heavy and persistent" in Dublin and other parts of southeast Ireland.
Most places will be dry at first but rain will spread northwards to all parts of the country this evening and tonight, finally reaching Ulster around midnight. The heaviest rain will be across the east and south of the country but snow is expected over Ulster on the higher ground pic.twitter.com/cGqImW5j8h
— Met Éireann (@MetEireann) 5 March 2019
Showers will be heavy and persistent for a time in the coming hours, with spot flooding possible.
The rain is expected to clear tomorrow but there is a chance of hail, thunder and frost on Wednesday night.
The forecaster added: "Clear spells and scattered showers on Wednesday night with a risk of hail and thunder.
"Light to moderate northwest breezes, will be fresh to strong and gusty on northern and eastern coasts. Lows of 1 to 4 degrees with some frost in places."
Meanwhile, temperatures could plummet to lows of -2C on Thursday night.
The full forecast for Thursday reads: "Cool, bright and breezy on Thursday, with sunny spells and passing showers, a few heavy. Blustery northwest winds at first will fall off moderate to fresh during the afternoon.
"Temperatures of 7 to 10 degrees Celsius are expected, but values may be a little lower in some northern parts. Cold and frosty overnight with lows of -2 to +3 degrees Celsius."
"Many areas dry, but a few showers will affect western and northern coasts. Wind falling light to moderate and backing southwesterly."