The status yellow high-temperature warning has been extended to run until Saturday morning.
It was due to run out tomorrow with the mercury expected to get as high as 30C nationally, and temperatures not to fall below 15C at night.
A similar orange alert for parts of Connacht, Leinster and along the border will run out tomorrow morning.
The weather warnings have been updated and the Yellow high temperature warning is now valid up to 9am Saturday! pic.twitter.com/v4GTvidgmq— Carlow Weather (@CarlowWeather) July 22, 2021
Wednesday was the hottest day of the year so far, after temperatures topped 30C in some parts of the country.
Mount Dillon in Co Roscommon recorded a temperature of 30.1C, according to Met Éireann, while a temperature of 30C was also recorded at Newport in Co Mayo.
The highest temperature of the year had previously been recorded on Saturday with 29.6C at Durrow, Co Laois.
The highest temperature ever recorded in Ireland was 33.3C at Kilkenny Castle, on June 26, 1887.
Meanwhile, the UK Met Office said Northern Ireland has, for the second time in five days, provisionally topped its all-time temperature record chart.
Castlederg in Co Tyrone recorded a temperature of 31.3C on Wednesday afternoon, exceeding the 31.2C recorded at Ballywatticock last Saturday.
While ice cream sales have soared during the good weather, many stores are being impacted by shortages of beach staples.
Buckets and spades, inflatables and swimming rings are becoming a rare commodity in shops across Ireland.
Galway shop owner Joe Quinn says supplies to his summer store in Salthill have been badly affected by the Suez canal blockage earlier this year.
He says getting delivers into the country has become a major issue.
"We have stock but we haven't got if we haven't got a full stock, we're out of all the big swim rings and out of the airbeds," he said.
"All suppliers are out of stock because of the ship that was in the Suez Canal, it was only released last Friday.
"There was 18,600 containers on it.
"They come into Ireland ... directly from China so that's affecting us all."
Mr Quinn also said that "Brexit is affecting everything as well" as stock supplies move through the UK to get to Irish shops.
A status orange high temperature warning is in place for Cavan, Monaghan, south Leitrim, Roscommon, Longford, Westmeath until 9am on Friday.
In these areas, maximum temperatures are likely to exceed 30C in places and overnight temperatures will drop to no lower than about 20C.
An orange heat warning is also in place for Antrim, Armagh, Down, Fermanagh, Tyrone, and Derry until Friday night.
A status yellow high temperature warning is in place for the rest of the country, with "hot conditions by day and staying very warm and humid at night", Met Éireann said.
Forecasters have said temperatures will continue to be hot for the rest of the week, with nights feeling “warm, humid and very muggy”.
Thursday will be another hot and mostly dry day, with plenty of sunshine, though it will be hazier than in recent days as high level cloud moves in.
Highest temperatures of 26C to 29C are forecast generally, reaching 30C locally.
Temperatures will be slightly lower along southern and eastern coasts, with moderate onshore breezes.
Another very warm day is forecast for Friday, with highest temperatures of 25C to 29C generally.
Scattered showers and thunderstorms will develop over the southern half of the country, with more isolated showers further north along with the best of the sunshine.
There is uncertainty around the weather over the weekend but current indications suggest Saturday will be slightly cooler than previous days with highest temperatures ranging from 18C to 20C in the east to 21C to 25C elsewhere.
High pressure still in charge so it’s staying very warm for the rest of this week, lots of sunshine but some ⛈️ possible too.— Met Éireann (@MetEireann) July 21, 2021
It won’t be quite as hot by the weekend, still warm though with 🌡️ in the low to mid 20s & more ☀️&⛈️
More in our News Story https://t.co/LCCLSbbsnu pic.twitter.com/PgP2xWj2Oq
Sunday will be a warm day with a mix of cloud, sunny spells and scattered showers.
Highest temperatures will be between 22c and 25C generally, a degree or two lower along the coasts, in generally light northerly breezes.
Similar temperatures are forecast in the east on Monday, becoming closer to average in the west, with this extending to all areas from Tuesday in a mostly north to northwest airflow.
Good sunny spells will be mixed with showers, some heavy at times.