Met Éireann has issued a status yellow weather warning for counties Donegal, Galway, Leitrim, Mayo, Sligo, Cork and Kerry.
The warning will come into effect at 12pm on Thursday 7 October for Cork and Kerry, and at midnight for counties Donegal, Galway, Leitrim, Mayo and Sligo.
The warning will be in place until the early hours of Saturday morning.
Met Éireann has forecast Friday to bring the heaviest and most persistent rain later in the evening and continuing into the night.
There will be prolonged rain with heavier bursts at times.
Significant accumulations of water are possible in upland areas and the heavy rain will pose a risk of localised flooding.