A status yellow rainfall warning has been issued for 10 counties.
Met Éireann has issued the warning for Galway, Leitrim, Mayo, Roscommon, Sligo, Donegal, Clare, Cork, Kerry and Limerick.
The warning comes into effect from 6am Friday and will be in place until 6am on Saturday.
The forecaster said: "Rain will turn heavy and persistent on Friday and Friday night bringing the risk of localised flooding.
"Rainfall totals of 30-50mm are expected during the period, higher in mountainous areas."
Met Éireann said the rest of the country will also see rain on Friday but it will be patchier.
Looking ahead to the rest of the weekend, Friday's heavy rain should give way on Saturday.
It predicts that "heavy rain from overnight [on Friday] will give way to bright or sunny spells and showers."
Those showers "will become confined to Atlantic coasts overnight on Saturday. "
Met Éireann predicts that Sunday will be "a fresh and cool day" that will see sunny spells and scattered showers.