Met Éireann has issued a yellow weather warning for Munster and Galway.
The rainfall warning is effective until 4am on Thursday.
It said showers on Wednesday night would merge into longer spells of thundery rain overnight which would carry a risk of spot flooding.
Rainfall of between 25 and 40 millimetres is possible.
The rain will clear to isolated showers by dawn.
Conditions are set to be drier in the east and north but with mist and fog.
Lowest temperatures will fall to around -1 to +3 degrees with frost in the east and north.
