The country is facing five days of wet weather, with heavy and thundery showers expected.
Motorists have been warned to be wary of spot-flooding on local roads while streams and rivers will be swollen and dangerous due to torrential rainfall.
Areas hit by flash floods over the past 72 hours will remain on high alert.
Temperatures will also dip from humid highs of 25C down to 19C as an Atlantic front pushes cooler air over the country.
Met Éireann said there is no sign of the unsettled conditions easing over the next week.
Today will be mostly cloudy, with early mist and fog clearing throughout the morning.
Heavy rain, with some heavy thundery bursts, will lash most of the country this afternoon, with Munster and Leinster expected to be worst hit.
Temperatures will range between 17C and 21C.
Further wet weather is forecast tomorrow, with long spells of rain through the morning. This will push northeastward toward the evening, bringing some drier conditions.
Highest temperatures tomorrow will be 18C to 22C.
As the week continues, however, there will be more rain and temperatures will drop as we head into the weekend.
Met Éireann said: "On Wednesday a band of rain will move northeastwards across the country, slowly clearing to sunny spells in the southwest by evening.
"The rain will be accompanied by moderate to fresh east to southeast winds, strong near coasts and highest temperatures of 18C to 20C."
There will be a mix of sunshine and showers on Thursday, with the heaviest in the south and west.
However, as we head into Friday, temperatures are expected to plummet.
The national forecaster said: "Friday will see a fresh to strong west to southwest flow expected to steer in further rain from the Atlantic.
"It will be getting cooler, with highs of 16C to 19C."