Met Eireann has issued a status yellow rainfall warning for Donegal, Cavan, Monaghan and Waterford, which comes into effect at this evening at 9pm and remains in place until 9am tomorrow (Saturday) morning.
All counties in Leinster are also covered under the warning.
Accumulations of 25-40mm of water are expected in some places.
Tomorrow showers remain in the north for much of the day.
The forecast for most of next week appears similar, with a mix of showers and some dry spells.
However, temperatures are expected to rise heading into next weekend.
The Road Safety Authority is urging motorists to watch out for flash flooding and to be aware of the risk of aquaplaning.
"Flooded roads could be deeper than you think so do not attempt to drive through them. Following the recent dry weather a build-up of oil and rubber deposit may have developed on the roads. After a long spell of dry weather these deposits mix with rainwater and the road becomes hazardous.
"The resulting greasy road surface increases stopping distances and the risk of a skid. The advice is to slow down and increase braking distance", it said.