A status yellow rain warning has been issued for almost half the country.
Met Eireann has put out the alert for all of Connacht as well as the counties of Longford, Louth, Westmeath, Meath, Cavan and Monaghan.
It will come in to effect at 9 o clock tonight and will last for 24 hours.
Heavy rainfall is being forecast during those hours which will lead to accumulations of 30 to 50mm.
The national forecaster says there may be some higher accumulations in mountainous areas.
It's warning this will bring a risk of some river and localised flooding.