A record May temperature of 28.8C has been recorded at two weather stations, according to Met Eireann data.
Killarney and Clonmel automatic climate stations recorded 28.8C at about 3pm on Monday.
The previous hottest May day was on May 31 1997 when a temperature of 28.4C was recorded at Ardfert Liscahane in Co Kerry.
Long spells of sunshine are expected throughout the country, the national meteorological service has said, with the highest temperatures largely between 23C and 28C.
Warm & dry with long spells of sunshine☀️
— Met Éireann (@MetEireann) May 25, 2026
While mist and fog patches will generally clear early on, some sea fog is possible through the day, especially along western and northern coasts🌫️
Highs of 22 to 27°C generally🌡️, warmest in Leinster & Munster, in light variable breezes pic.twitter.com/kP5nr1xcGe
The warm weather is expected to continue, with temperatures of up to 28C again forecast for Tuesday and 27C on Wednesday.
The UK had its hottest May day with a high of 33.5C in Heathrow, west London.
Met Office observations also put parts of Northern Ireland between 24C and 25C after lunchtime.
Meanwhile, the Irish Cancer Society reminded people to protect their skin from harmful ultraviolet rays – the main cause of skin cancer.
Michelle Lonergan, cancer awareness nurse at the charity, said: “Over-exposure to the sun can damage the skin at any age, and this damage builds up over time, increasing the risk of melanoma and other types of skin cancer.
“Even on cloudy or cooler days, UV rays can still penetrate the skin, making daily protection essential whenever you are outdoors.”
Ms Lonergan said simple steps can make a “huge difference in reducing” risk – including regularly reapplying a broad-spectrum sunscreen with a high SPF (30 or above), and wearing lightweight long sleeves, sunglasses and a wide-brimmed hat.
She also said it was important to protect babies and children as their skin is more sensitive to the sun.
