A status yellow rain warning has been issued for five counties.
The warning will affect Carlow, Kilkenny, Wexford, Wicklow and Waterford, and take effect from midday on Thursday to 3am on Friday.
Met Éireann issued the warning on Wednesday morning. Affected counties will experience spells of heavy rain with a possibility of thunderstorms, the forecaster said.
Potential impacts include localised flooding, disruption to outdoor events, and difficult travelling conditions.
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Met Éireann also warned of unsettled conditions on Wednesday, possibly bringing heavy rain and thunder.
Dublin will be mostly cloudy, with rain becoming light and patchy in the morning.
In the afternoon and early evening, scattered showers will fall, though sunny spells will develop later on in the day. Highest temperatures will reach 18 to 23 degrees.
On Wednesday night, there will be showers or longer spells of rain early in the night, with a continued chance of thunderstorms before clear spells develop.
Across the rest of Leinster, conditions will remain similar; however, on Wednesday afternoon and early evening, some showers will be heavy, with possible isolated thunderstorms.
The national forecaster said Ulster will have a similar start, with cloudy conditions and rain becoming light and patchy.
There will be some scattered showers in the afternoon and early evening, with some turning heavy. Highest temperatures will reach 18 to 22 degrees.
Connacht will also experience cloudy conditions and some patchy rain in the morning, followed by scattered showers in the afternoon and early evening. Some of the showers will turn heavy, with isolated thunderstorms also possible, and highest temperatures of 19 to 23 degrees.
Met Éireann said Munster will have the same weather conditions; however, it will experience the highest temperatures across the country on Wednesday, with highs of 20 or climbing to as much as 23 degrees.
Nationally, Thursday will have a dry start in many areas.
Throughout the day, rain in the south and southeast will extend northwestward to most areas, heavy at times, with isolated thunderstorms and surface flooding possible, and highest temperatures of 18 to 24 degrees.
On Thursday night, showers will become more isolated and patches of mist or fog may form.
Friday will bring bright or sunny spells, scattered showers, and temperatures of about 18 to 23 degrees, though warmest in the southeast.
Rain will develop in the west and spread eastward overnight.
