Four weather stations across the country have provisionally recorded heatwave conditions, Met Éireann has said.
The forecaster defines a heatwave as when a temperature over 25C is recorded at a weather station for five or more days in a row.
It said the highest temperature today was 28.2C at Oak Park, Co Carlow.
Four Met Éireann stations have provisionally reached heatwave status today. 📈 🌡️— Met Éireann (@MetEireann) September 8, 2023
A heatwave occurs at a Met Éireann weather station when its daily maximum shaded air temperature is greater than 25°C for 5 or more consecutive days. 🗓️☀️ pic.twitter.com/ZYj4cs9QMU
Met Éireann added it was not the first time a heatwave was recorded in September, saying that heatwaves were recorded at Kilkenny Castle in 1959 and 1991.
The hottest day recorded for the month was at Clongowes Wood College, Co Kildare in 1906.
In a post on social media site X, formerly Twitter, Met Éireann said: "This year's heatwave hasn’t broken the record for the moment and at this stage it is not expected to break it, as temps are forecasted to drop tomorrow."
A Status Yellow heat warning remains in place for Ireland today with daytime temperatures of over 27C predicted for many areas.
The warning, issued Wednesday, is in place until 8am tomorrow.
Nighttime temperatures will not fall below 15C, Met Éireann said.
The forecaster warned of the risk of heat stress and uncomfortable sleeping conditions.
Today was very warm with highest temperatures of 24C to 28C, Met Éireann said.
A few scattered thunderstorms may develop inland during this afternoon.
Summer 2023 set new records in terms of heat and rain and is further evidence we are living at a time when our climate is changing, according to Met Éireann.
Ireland had its warmest June ever, followed by the wettest ever July, and rounded off by flash flooding and downed trees in August with Storms Francis and Betty wreaking havoc.
Authority appeals for caution on the water
Water Safety Ireland is renewing an appealing to people who will be doing water-based activities to take all the necessary precautions to ensure they stay safe.
It comes after two more drownings this week, including a girl in Co Cork just days before her eighth birthday. Figures show that around nine people per month drown in Ireland and 60% will occur in inland waterways.
People who are planning to go to a waterway this weekend are being advised to visit lifeguarded water, Joanne Walsh of Water Safety Ireland said.
Speaking on RTÉ's Morning Ireland, she said: "If you see a red flag, do not swim. That means the lifeguard has considered it dangerous to swim, so please heed the advice of our lifeguards."
Further information can be found here.
She said if people want to swim, could do so at a traditional bathing area, which is known to be safe.
"Check local knowledge. A local shop, the local gardaí, a local ice-cream van.
"When you're there, please check the safety signage ... it might be identifying any hazards.
"Swim in an area where there is a lifebuoy present that somebody can gain access, to help you if required."
Ms Walsh said inflatable toys are "a real threat to life" on the water and should not be used.
"Over the last ten years, unfortunately in Ireland we've had, on average 105 drownings," she said.
"The majority of drownings are males ... and approximately 60% will happen inland.
Alcohol is a factor in approximately 30% of drownings, Ms Walsh said.