The warning applies to Clare, Cork, Mayo, Kilkenny, Limerick, Laois, Offaly, Galway, Kerry and Tipperary.
Met Éireann has issued public safety advice on coping with the high temperatures, highlighting the importance of water safety, staying cool while sleeping, being aware of the increased forest fire risk, and protecting animal welfare.
A nationwide weather advisory also remains in place due to a very high UV index, with an increased risk of heat stress, sunburn and other sun-related health issues.
An Garda Síochána is urging motorists to slow down and allow extra time for journeys, as roads are expected to be busier than usual with people travelling to outdoor events, beaches, lakes and parks.
The hot weather is forecast to continue until at least Thursday.
