A status yellow weather alert for high temperatures has been issued for Leinster and Munster.
Met Éireann say it will be "very warm or hot" across Thursday, Friday and Saturday.
Maximum temperatures will be between 27° and 29°.
It will also be quite warm at night, with temperatures generally staying above 15°.
However it says daytime temperatures may not be as high in coastal areas due to sea breezes.
But it is warning specifically of:
- Heat stress, especially for more vulnerable sections of the population
- A high solar UV index
- A risk of water-related incidents
The alert is in place from midday Thursday until 6am on Sunday.
A Weather Advisory is already in place for the entire country for hot weather.
Generally, Wednesday will also be a very warm day - with highest temperatures of 23° to 26°.
These will touch 27° in a few localised parts of Leinster and Munster.
It will be a little cooler in coastal parts of the northwest, with a more moderate southwest to west breeze.