Met Eireann has issued a status yellow rainfall warning for Donegal, Leitrim, Mayo and Sligo.
Heavy rain and blustery conditions will be expected when the alert comes into effect at 9 o'clock tomorrow morning.
There will be a risk of spot flooding with up to 6mm of rain expected to fall in places.
Meanwhile, Irish Water says thunder and lightning earlier today has had an impact on supplies in South Letterkenny, with Crieve, Bomany, Leitirleague, Foxhall, Conwal and Roughan affected with low water pressure.
They say the drinking water treatment plant at Goldrum is operating as normal, but it may take some time for the reservoir levels at Rahn to replenish, and some customers may experience intermittent low water pressures over the weekend.