Temperatures are expected to reach 22 degrees Celsius today.
The best of the weather is in Ulster and Connacht, but these temperatures will lead to thundery showers later.
Hazy sunshine is forecast tomorrow, with temperatures of between 18 and 20 degrees.
However, we can expect fresher weather by mid-week.
May's high temperatures follow on from the coldest and dullest April in decades.
In London, temperatures climbed to just over 27 degrees yesterday.