Met Éireann has announced a status yellow rainfall warning for Munster and Leinster.
The warning will come into effect on Thursday, August 8, at 7pm.
It will remain in place until 7am on Friday.
Met Éireann say heavy rain "will spread from the south Thursday evening and night.
"Accumulations of 25 to 40mm will occur over a relatively short time period. Spot flooding is likely."
Wednesday will stay mainly dry with just a few isolated showers, according to the forecaster.
They say they showers will become "more widespread this afternoon and evening" with some "heavy and thundery bursts, particularly over midlands, north and east."
It should dry up tonight and Thursday is set to start dry with good sunny spells.
However, cloud will begin to develop before the rain begins to set in.