Met Éireann has issued a status yellow rainfall warning for four western counties.
The alert is in place for Donegal, Galway, Mayo and Sligo from 3.00pm on Friday.
The forecaster says there will be heavy rain with accumulations of 40-50mm and localised flooding.
At the coast, there will be high seas and strong to gale force southerly winds - which will see a risk of coastal flooding.
Friday will see a cold, crisp and frosty start with good spells of sunshine at first.
But cloud will thicken from the west through the morning in a freshening southerly breeze.
Rain will then push into Connacht, Ulster and north Leinster through the day - with patchier rain in the south.
It will become increasingly windy as the day goes on, with strong and gusty southerly winds countrywide by the evening.
There will be highs of 9 to 11°C.
And Friday night will be wet and windy, with the rain turning persistent and heavy at times along Atlantic coasts.
Southerly winds will continue to be fresh to strong and gusty with gales.