Monday was officially the hottest day of 2018 as the temperature hit 27.5C in Carlow.
The heat shows no sign of letting up today, though Met Éireann has said there will be a few isolated showers in some areas of the country.
A nationwide warning was issued at 6am on Monday and will be valid until 9pm on Friday.
Irish Water has warned that it may have to implement water restrictions as the country is set to swelter in a 30C heatwave this week.
The utility has urged householders and businesses to conserve water as supplies are now at serious risk in the greater Dublin area, as well as Donegal and the midlands, particularly in Athlone and Mullingar, parts of Offaly and Kilkenny.
The Dublin region alone is currently using 600 million litres of water a day, which is close to the maximum level of water that can be treated on a daily basis.
Irish Water may introduce night-time water restrictions.
It was also the hottest day of the year across the water in England as temperatures soared to over 29.4C in London.
Sunseekers on both sides of the Irish Sea have been warned that pollen levels will continue to remain at very high levels.
"If demand consistently meets or exceeds supply, there may be water outages in the greater Dublin area as Irish Water manages the network and tries to minimise the impact on homes and businesses.
"Irish Water's drought management group has been meeting regularly to assess at-risk supplies that include the midlands - particularly Athlone and Mullingar - parts of Offaly, Kilkenny, parts of Donegal and the greater Dublin area, where demand for treated water almost meets supply availability regularly," Irish water said in a statement.
Currently, the supply of untreated water in reservoirs serving the greater Dublin area - the Varty reservoir at Roundwood and the Poulaphouca reservoir in Co Wicklow - are "reasonable and we do not see shortages at present".
However, if the current dry spell continues, "they may come under pressure later in the year".
"The main pressure is on treated water capacity, which, along with our treated water storage, can meet the normal daily range of demands at present.
"However, in the recent dry spells, we have seen a significant increase of the order of 20-30 million litres per day, which reaches and occasionally exceeds our maximum production capacity. This is not sustainable on a long-term basis without some management action," said the utility.
Met Éireann has said temperatures are set to climb steadily in the coming days as the heatwave takes hold.