Seven counties have been hit with a Status Yellow rainfall warning with up to 32mm of rain expected to fall in the next 24 hours.
Met Eireann issued the alert on Wednesday morning as the unseasonable heavy showers show no sign of stopping.
Cavan, Monaghan, Donegal, Leitrim, Mayo, Roscommon and Sligo have been issued with the caution.
The yellow alert was valid from 9am on Wednesday and will last until 9am on Thursday.
A Met Eireann spokesperson said: "Another 25 to 32 mm of rainfall expected over 24 hours."
The counties in the north west are to be lashed by rain on Wednesday night.
However, counties in the south are expected to experience a clear and dry night. Lowest temperatures may plunge to as low as 2C.
The spokesperson added: "Further heavy rain overnight - again especially over the north and west. Good dry spells over southern counties with clear spells.
"Lowest temperatures ranging from a cold 2 degrees during clear spells in the south to 7 or 8 in the north.
"Some fog patches inland and in the south. Moderate northwest winds."
