Eight counties have been hit with a Status Yellow rainfall warning.
Met Eireann has issued the alert for counties in the south and southeast as torrential rain on Sunday night is set to cause flooding in many areas.
The caution will be in place for Carlow, Kilkenny, Laois, Wexford, Wicklow, Cork, Tipperary and Waterford.
The weather warning will be valid from 8pm on Sunday to 10am on Monday.
A spokesman said: "Heavy rain tonight leading to accumulations of 20 to 30mm in places, with a risk of spot flooding."
All parts of the country are to experience rainfall on Sunday night but the south and southeast will get the worst conditions.
Lowest temperatures will range from 9C to 13C.
A Met Eireann forecaster said: "Rain will push northwards to all areas tonight, turning heavy and persistent with a risk of spot flooding in the south and southeast.
"Minimum temperatures of 9 to 13 degrees, coolest in north Ulster. Moderate variable winds, becoming fresh to strong on the south coast later."
It will be a brutal start to the week weather wise for many areas as Met Eireann is predicting heavy and torrential downpours, which may spark flooding on Monday morning.
A spokesman said: "A wet start on Monday morning with outbreaks of rain, heavy and persistent in the east and southeast with a risk of spot flooding.
"However, drier and brighter weather will quickly extend from the west with just well-scattered showers and sunny spells for the afternoon and evening.
"Highest temperatures of 16 to 20 degrees, warmest in the southeast with a moderate to fresh northwest breeze developing."
