Update 5pm: Ireland has recorded its hottest temperature in 42 years.
Met Éireann says the mercury has reached 32 degrees Celsius at Shannon Airport this afternoon.
It has not been that hot here since 1976.
Around 100 water supply schemes across the country are now at risk.
Irish Water says it's lowering night time pressure in the greater Dublin area to its lowest possible level as demand in Dublin is exceeding supply.
The utility says as well as relying on its reserves it is also looking at reconfiguring the supply in some areas to try to avoid outages.
Kate Gannon from Irish Water says there are also ways the public can help:
"Just stop using water outside the home, stop hosing plants, avoid paddling pools where you can, don't wash your cars.
"We're asking people to look at any area they can conserve water in their home, in their business and in their communities.
"We have lots of tips on water.ie. If you see a running leak call us on 1850 278 278.
People are being urged to stay away from forest and gorse fires.
Dublin Fire Brigade has dealt with around 200 fires so far and says conditions can change quickly and people can become stuck.
This led to the rescue of one family in the Dublin mountains yesterday.
Unfortunately a car was destroyed in today's gorse fire (Dublin mountains) after a change in wind direction ruined visibility on the road. Smoke from these fires affects your breathing & ability to see, enjoy the ☀, be safe #Heatwave #HeatwaveIreland pic.twitter.com/Yjjj4mtWL6— Dublin Fire Brigade (@DubFireBrigade) June 27, 2018
Darren O'Connor Sub Officer with Dublin Fire Brigade says they are extremely dangerous:
"Yesterday there was a car that was lost in one of the fires up on the Dublin/Wicklow border.
"We would appeal to people not to come and look at these. The wind can change directiin extremely quickly. Do not come and look at them.
"They can burn down towards the roadways, you become disorientated if you get caught in the smoke. We're asking people to just stay away."
Latest 12.46pm: People in Dublin could be facing water outages and restrictions as the demand in Dublin is now more than can be supplied.
Demand is now at 615 million litres but the maximum that can be produced is 610 million.
Around 10,000 people across the country are currently having their water restricted at off-peak times but this could increase further.
Kate Gannon from Irish Water says the situation is being closely monitored.
"Well, our operational teams are meeting on the ground at the moment, and they are looking at all available options such as reconfiguring the network and looking at different ways to supply people," she said.
The high temperatures continue to cause fires as thousands of acres of land have been damaged by fires in the past few days.
Wow - this is the gorse fire that firefighters have been tackling near the Glenshane Pass in County Derry. Photos by Kevin McCloskey. #Heatwave pic.twitter.com/7NzOPHFMRp— Barra Best (@barrabest) June 28, 2018
Since Monday, there have been four forest fires, 44 gorse fires, 147 grass fires in Dublin and the surrounding counties.
There are 11 incidents ongoing and three units of Dublin Fire Brigade have been out this morning.
National Weather Warnings
Status: Yellow pic.twitter.com/vKTIKlEncv— Met Éireann (@MetEireann) June 28, 2018
Meanwhile, Met Eireann has explained why car thermometers might be giving a false reading of the temperature.
The weather is officially set to be called a heatwave today once the mercury reaches 25 degrees, though many people feel like it is much warmer.
Forecasters use Stevenson screens at climate stations so they're only measuring the air temperature and not the direct sunlight.
A number of water schemes across the country are in a critical situation as the heatwave continues.
Reservoir levels have dropped and there is also extra demand on the system as people are using more water.
In order to allow water levels to recover at the Wolfhill Reservoir in Co. Laois, water restrictions will be in place for customers in The Swan, Wolfhill, Mayo & Doonane tonight and tomorrow night. See https://t.co/RsjWFb8hzo for more. #ConserveWater #irishWater— Irish Water (@IrishWater) June 28, 2018
Parts of Leinster are officially in drought, while two areas in Connacht are having to rely on a backup supply.
The drought is causing significant concern for farmers who are worried they won't be able to save enough fodder for next winter as they try to catch up with the wet spring this year.
Seamus Granahan from Irish Water is asking customers to reduce usage and conserve water.
"Rural areas and urban areas, both have increased demand," he said.
Irish Water is being urged to prosecute homeowners who do not fix leaks on their properties.
Green Party Councillor Ciaran Cuffe says the heatwave is making it more important than ever to deter people from wasting supplies.
"I had a look at the legislation and the Water Services Act 2007 says Irish Water can prosecute if somebody is just allowing water to be wastefully leaked so I think Irish Water need to up their game a little bit," he said.
The high humidity levels are also creating the ideal condition for forest fires.
There are warnings of gorse fire after a family was rescued from the Dublin Mountains last night.
Firefighters have had to tackle a number of gorse fires over the past few days and Mick Power from Coillte has warned that the smallest spark can start a fire.
"We're appealing to people today to be extremely careful during this hot spell," he said.
Helicopters passing overhead with water containers since early morning. Knackered, soot covered, fire fighters speeding around, sirens blaring.
I always wanted to live in California. pic.twitter.com/EUve1lhYMC— Philip Boucher-Hayes (@boucherhayes) June 28, 2018
Meanwhile, Irish Rail is handing out 50,000 bottles of water to commuters during the heatwave as part of its hot weather protocols.
It is advising people to avoid unnecessary travel if sick, to wear comfortable clothes and not to sit in the direct sunlight.
Irish Rail says it is ensuring that air conditioning is operating well, while fleets are being monitored.
By Eoin English and Evelyn Ring
The country continues to sizzle in the sun with temperatures expected to exceed 30 degrees today and tomorrow.
Met Éireann forecaster John Eagleton said the best of the sunny weather would continue until Saturday before cooling off slightly.
“There was some rain expected on Sunday and that would have been welcomed by a lot of people but it does not look as if it going to happen now,” said Mr Eagleton.
“Temperatures could reach 30-plus today and tomorrow and we are expecting another warm and sunny day on Saturday.”
Mr Eagleton said the humidity — the amount of moisture in the air — was low, and that was helping to keep the weather more bearable at night.
“But if it was humid you would not be getting the temperatures we are experiencing at the moment,” he pointed out.
The meteorologist said it looked like it would get a bit cooler next week.
“On Sunday temperatures will start to drop back but they will start to rise again next week.
“It will be warm next week but not as warm as it has been towards the end of this week.”
Met Éireann’s status yellow high-temperature warning remains in place until 8pm tomorrow.
Irish Water has said demand for water in the greater Dublin area remained “critically high” but there was some evidence that people were beginning to conserve water.
In many of Irish Water’s schemes around the country supply and demand is under pressure and the warm weather is exacerbating the situation.
Some areas in Athlone, Kilkenny, Longford, and north Dublin have experienced outages and restrictions and areas in Donegal, Galway, Limerick, and Mullingar have been identified as being at risk.
Kate, our Corporate Affairs Manager & Water Conservation expert, outlines how the current prolonged dry spell ☀️ impacts our water supplies. Small measures benefit you, your neighbours & your community. See more on this at https://t.co/2iJ1KfUDGU #BeWaterSmart #ConserveWater pic.twitter.com/ygC555Pp1o— Irish Water (@IrishWater) June 25, 2018
Irish Water’s corporate affairs manager Kate Gannon warned that if the drought was prolonged and demand did not continue to drop, water restrictions would become unavoidable.
“The situation remains critical and we are continuing to seek the public’s help,” said Ms Gannon.
Meanwhile, firefighters in Cork have repeated warnings about the dangers of spontaneous fires during the heatwave after tackling a large hay barn blaze north of Cork city yesterday.