More than 50mm of rain will fall in parts of Munster today, according to Met Éireann.
An orange weather warning is in place in Tipperary and Waterford until 6pm this evening.
"Heavy rain during the period will cause spot flooding," said the forecaster.
"Accumulations in excess of 50mm possible in some parts."
Spot flooding is also forecast in Kilkenny, Wexford and Cork, where there is a yellow warning in place until the same time.
Meteorologist Cathal Nolan from the Midland Weather Channel says a very wet day is in store.
"It's certainly the southern counties through Munster that are going to bear the brunt of that rainfall at present, with an amber weather warning in place across counties Waterford and Tipperary," said Mr Nolan.
"Rainfall totals in those locations are expected to exceed 50mm, possibly reaching up as high as 70mm across some of the more mountainous areas.
"In the surrounding counties across parts of Munster and into south Leinster we do expect to see some heavy falls of rain also, with totals reaching up to 50mm."
Met Éireann said that Friday will "continue mostly cloudy" and will see "outbreaks of rain in most areas".
They added that it will continue to be "unsettled through the end of the week and weekend" adding that there will be "some heavy rain at times".
Meanwhile, drivers are being warned of dangerous driving conditions today because of very heavy rain.
Mr Nolan said there will be "quite a large total" of surface water in places and he also warned of "isolated flooding events" in Munster and south Leinster.