A status yellow rainfall warning has been issued for Munster and Leinster.
Met Éireann says rain will move in from the south this afternoon, with up to 35mm expected.
The warning is in place from 3pm until 6am tomorrow.
"We are in an unsettled spell of weather as you have seen in the last few days," meteorologist Harm Luijkx told.
"There will be heavy rain moving up from the south this afternoon.
"It will mostly affect the south and east of the country. The rain will start, at the latest, this afternoon.
"It will be heavy in the evening and early tonight and will clear before tomorrow morning," he added.