Met Éireann has issued a status yellow rain warning for Galway which will come into force at midnight tonight, remaining in place until Saturday morning.
Prolonged rain with some heavier downpours are forecast across the West, with the heaviest and most persistent rain later on Friday.
Significant accumulations are possible in upland areas, with a risk of localised flooding in places, Met Éireann says.
The warning, which is set to last until 3am on Saturday morning, also affects Donegal, Leitrim, Mayo and Sligo.
Today, Wednesday, will stay cloudy with outbreaks of rain and drizzle and some heavy rain in the West – but drier weather will then develop in all areas.
It will remain very mild and humid, with patches of mist and fog on hills and in some coastal areas.
Thursday will be another dull, humid and breezy day with outbreaks of heavy and persistent rain.
It is set to be another warm day, with highest temperatures of 17 to 19 degrees in moderate to fresh southerly winds.