Met Éireann has issued a status yellow rainfall warning for counties in Munster and Connacht.
Heavy showers or longer spells of rain with thundery downpours in some places may lead to spot flooding, it said.
The warning will come into effect at 12pm on Friday and will remain in place until 12pm on Saturday.
On Friday, rain is forecast to spread northwards in the morning and afternoon in Munster and Connacht. This will be followed by showers for the evening.
Persistent rain is forecast across Leinster on Friday afternoon and evening. This will be followed by isolated showers.
In Ulster, the day will be mostly cloudy with scattered showers, followed by more persistent rain the late afternoon and evening. There is a risk of localised flooding too.
The weekend is forecast to be unsettled and cooler, with rain and heavy showers at times.