Met Éireann has issued a status yellow rain warning for six counties, lasting for a 24-hour period.
The warning for Clare, Donegal, Galway, Leitrim, Mayo, and Sligo will be in place from 6pm Wednesday until 6pm Thursday.
Met Éireann warned of “significant rainfall accumulations from heavy showers and longer spells of rain”.
Isolated thunderstorms are also possible.
The potential impacts include localised flooding, poor visibility, and difficult travel conditions.
More generally, Met Éireann said thundery rain early Wednesday will give way to sunny spells and scattered heavy showers.
Most showers will affect Atlantic counties, merging into longer spells of rain with local lightning. By Wednesday evening, some showers will extend further east.
Temperatures today will range from 16C to 21C, warmest in Leinster, with mostly moderate to fresh southwest winds.
Wednesday night will bring scattered heavy showers, merging into longer spells of thundery rain across Atlantic counties, with spot flooding. Conditions will be drier and clearer further east. Temperatures will fall to 10C–13C in mostly moderate southwest winds.
On Thursday, low pressure will move across the country, bringing blustery southwesterly winds and frequent showers, merging into longer spells of rain with local lightning and spot flooding, especially in the northwest.
Some sunnier and drier conditions are expected in the southeast, where it could remain dry. Temperatures on Thursday will be slightly cooler, at 15C to 20C.