Temperatures could hit highs of 27 degrees with a risk of forest fires.
Carlow, Kildare, Kilkenny, Laois, Longford, Offaly, Westmeath, Cavan, Donegal, Clare, Limerick and Tipperary are among the counties affected.
The alert will last until 6 o'clock this evening.
Meanwhile, Connacht will come under a thunderstorm warning from 3pm to 10 o'clock tonight.
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