Up to 70mm of rain could fall in parts of the country tonight (Thursday) and Friday, Met Eireann has said.
It has issued an orange rainfall warning for Connacht, Donegal, Clare, Cork, Kerry, Limerick and Tipperary until midnight on Friday.
Widespread, very heavy rain, with some thundery downpours will develop Thursday night and persist through Friday, it said.
Met Eireann said this will lead to spot flooding and possibly accumulations of 35 to 50mm, but up to 70mm possibly on hilly ground and on mountains.
Met Eireann has also issued a status yellow rainfall warning for Leinster, Cavan, Monaghan and Waterford until early Saturday morning.
Heavy falls of rain will develop during Friday and Friday night, leading to spot flooding, possible accumulations of 25 to 35mm possible, it said.
The Road Safety Authority (RSA) is advising all road users to take extra care when using the roads as Met Eireann warn of widespread and very heavy rain in Connacht, Donegal, Clare, Cork, Kerry, Limerick and Tipperary.
The poor weather will create potentially hazardous conditions for road users. As such the RSA is advising drivers that:
The RSA also has the following advice for driving in flooded conditions: