A status yellow rainfall warning has been issued for five counties for Tuesday night into Wednesday.
The alert is in place for Galway, Mayo, Cork, Kerry and Waterford.
Between 30 to 50mm of rain is forecast with higher amounts in mountainous areas.
There is also a warning of flooding in places - as well as reduced visibility and hazardous driving conditions.
The alert is in place for 24 hours from 6.00pm on Tuesday.
Overall, rain will become widespread on Tuesday night with heavy falls - especially across western and southern counties.
It will become windy too, in fresh to strong southerly winds.
But it will be relatively mild with lowest temperatures of 9 to 11 degrees Celsius.
Wednesday will be very wet and windy with an ongoing risk of flooding and reduced visibility.
During the afternoon the rain will ease off across the west and southwest, but will persist across eastern counties into the evening.
There will also be strong and gusty southerly winds with highest temperatures of 11 to 13 degrees Celsius.