A Yellow – High Temperature warning will take effect at midday today for Connacht and 12 counties.
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 6:00pm this evening.
Meanwhile, Connacht will come under a thunderstorm warning from 3:00pm to 10:00pm tonight.