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.
But we can expect fresher weather by mid-week.
May_ s high temperatures follow on from the coldest and dullest April in decades.
As the temperatures climb this week, people are being reminded about their safety when swimming in lakes and rivers.
Irish Water Safety says around 135 drownings take place annually.
Spokesperson John Leech says anyone planning a swim should take extra precautions.
