A status yellow rainfall warning has been issued for 10 counties.
The warning comes into effect tomorrow morning at 6am, and will remain valid for 24 hours.
It covers all of Connacht, as well as Donegal, Clare, Cork, Kerry and Limerick.
Status Yellow- Rainfall warning issued for Connacht, Donegal, Clare, Cork, Kerry and Limerick
Valid from 06:00 hours Fri, 30-Aug-2019 until 06:00 hours Sat, 31-Aug-2019 pic.twitter.com/7aaOKY151N
— Met Éireann (@MetEireann) August 29, 2019
Met Éireann says rain will turn "heavy and persistent" on Friday and Friday night, with a risk of localised flooding.
Up to 50mm of rain is forecast to fall, with higher amounts in mountainous areas.
Tomorrow's national weather forecast says outbreaks of rain are expected to extend to most of the country.
Outside the warning areas, forecasters say the rain is expected to be "patchier... with decent dry periods".
The rain on Friday night is expected to give way to bright or sunny spells and showers on Saturday morning.