A Status Orange rain warning is now under way for counties Cork and Waterford, with six other counties under Status Yellow rain warnings throughout the night.
The orange warning is now in effect until 5am tomorrow morning.
Met Éireann said there will be heavy rain and possible thundery downpours.
The forecaster said this could lead to river flooding, especially in mountainous areas, and difficult travelling conditions throughout the two counties.
Met Éireann senior forecaster Gerry Murphy said that there is a very heavy band of ran moving up over the country tonight but gradually clearing tomorrow.
"The most significant warning is the orange level warning for Cork and Waterford. In these areas there, in all of the regions, there be the potential for surface flooding. In many places, difficult traveling conditions in that heavy rain, the possibility of thunderstorms there as well," he said.
Mr Murphy also stressed the possibility of river flooding, especially on higher ground in mountainous areas, and most especially in West Cork and possibly East Kerry as well.
A Status Yellow rain warning for Carlow, Kilkenny and Wexford is now in place until 8am tomorrow morning, while a Status Yellow rain warning is also in place for Kerry until 5am tomorrow.
The national forecaster also added a Status Yellow rain warning from 11pm tonight until 9am tomorrow for county Wicklow.
While Louth will be under a Status Yellow rain warning from midnight until midday tomorrow.
With regard to the yellow warnings, Met Éireann is warning of spells of heavy rain with the possibility of a few thundery downpours.
It said possible impacts include localised flooding and difficult travelling conditions.
A Status Yellow rain warning has been issued by the UK Met Office for counties Antrim, Armagh and Down from midnight tonight until noon tomorrow.
Additional reporting by Jackie Fox
