Met Eireann has issued a Status Yellow rainfall warning for nine counties as up to 40mm of rain is set to lash the west, northwest and southwest.
And the national forecaster warned local flooding is likely due to the "saturated" ground and elevated river levels following Storm Dennis.
The alert covers all of Connacht as well as Longford, Donegal, Clare and Kerry.
It comes into effect at 4am on Wednesday morning and will last for 24 hours.
A forecaster said: "Heavy rainfall through Wednesday into Wednesday night, with 25mm to 40mm expected.
"Localised flooding is likely as ground is saturated at the moment and river levels are elevated following recent heavy rainfall."
And the weather shows no sign of improving in the immediate future as forecasters predicted a chilly Tuesday with scattered rain and hail showers.
A forecaster said: "Cold and blustery today, with sunny spells and scattered showers, mainly of rain or hail, with a risk of thunder.
"Showers will be heavy and prolonged in places this morning, merging to give longer spells of rain.
"A few isolated wintry showers of sleet or snow are possible, mainly over high ground.
"Maximum temperatures 5C to 8C, in fresh to strong and gusty westerly winds, up to gale force on Atlantic coasts."
Temperatures are set to plunge to as low as 0C overnight, with frost and icy patches expected to form.
But showers and winds won't be as heavy and strong.
The forecaster said: "Early tonight, showers will largely die out and most areas will be dry for a time, with clear spells and winds easing.
"Cloud will increase from the Atlantic and outbreaks of rain will extend eastwards across the country, reaching all but northern and eastern areas by morning.
"Cold at first, with minimum temperatures of 0C to 4C, with some frost and a risk of icy patches, but temperatures will rise later.
"Winds moderate westerly, later backing southerly in Atlantic coastal counties and freshening."
