Update 9.50pm: Temperatures reached a high of 27.5 degrees today with Met Éireann predicting even hotter weather is on the way.
The peak was at Oak Park in Carlow, while 27 degrees was recorded at Shannon Airport and 25 degrees at the Phoenix Park in Dublin.
A status yellow high-temperature warning is in place for the whole country, with forecasters expecting it to be the hottest warm spell since 1976.
The current warm weather is expected to continue until after the weekend.
Update 5.10pm: Water supplies in some parts of the country are already under pressure due to the good weather.
Irish Water is appealing to people to conserve water as a prolonged dry spell has been predicted by Met Eireann.
Dropping water levels in lakes and rivers means less water to supply homes and businesses.
Irish Water's Corporate Affairs Manager, Kate Gannon says they are monitoring the situation:
"Current storage in the greater Dublin area and across the country are reasonable at the moment," she said.
"We're not seeing immediate restrictions at present but if the summer and autumn continue to be very dry and hot our raw water sources, i.e. the rivers and lakes, will come under pressure later this month and later in the year.
Update 3.20pm: The charity Alone is asking members of the public to check in on their elderly neighbours during this week's heatwave.
Alone is urging older people to take extra care as temperatures are expected to approach 30 degrees this week.
Met Eireann has forecast hot weather conditions for the coming week, with overnight temperatures reaching low to mid-teens.
Alone are encouraging older people to stay hydrated, avoid direct sunlight for extended periods and wear lighter clothes made of natural fabrics.
"Older people, and their families and neighbours, should be conscious that they are more at risk of dehydration," said Seán Moynihan, CEO of Alone.
"What’s more, factors such as poor circulation, heart, lung or kidney problems can also heighten the dangers.
"We ask that everyone remembers their older neighbours living alone and to consider their needs, especially those with health issues, and those with limited mobility.
Further information, including advice and tips for older people in adverse weather conditions, can be found here.
Update 2.20pm: Ireland could experience the hottest summer since 1976 with a heatwave forecast for this week.
A status yellow high-temperature warning is in place nationwide with highs of 27 degrees expected in some parts today.
Homeless services are appealing for donations of water and sunscreen while people are being urged to check on elderly neighbours.
Homeless charities are warning that rising temperatures could pose a risk for rough sleepers.
Inner City Helping Homeless has had hydration teams out this morning and outreach teams will be out every night as normal.
Anthony Flynn, CEO of Inner City Helping Homeless says they are worried about people stranded outside without shelter, as well as families in emergency accommodation.
"Normally we would be warning in regard to cold weather but unfortunately this week, or fortunately enough this week, we have a lot of hot weather and there's grave concern for the individuals who are on the street and spending excessive amounts of time on the street - so rough sleepers and homeless people," said Mr Flynn.
"At the moment we're looking at putting hydration teams together to make sure that people on the streets are being hydrated.
"There is a fear of dehydration or sunstroke for people who are spending excessive amounts of time on the street."
Mr Flynn told Sean O'Rourke on RTÉ Radio 1 that hydration stations will be open outside hospitals and homeless facilities.
Forecaster with Met Eireann Harm Luijkx says it has been a year of extreme weather, but there's no connection between the different events.
"It's a coincidence. It is not a typical weather year, that's for sure.
"We have had good weather in May and most of June had good weather. That makes it more exceptional compared to other years. It is the length of the spell of good weather really."
This week's heatwave could result in the hottest summer in Ireland since the seventies.
Met Éireann is forecasting temperatures between 24 and 27 degrees on Monday and remaining high into the evening with lowest temperatures of 12 to 16 degrees.
Tuesday looks set to follow a similar pattern and will be very hot with spells of hazy sunshine. Top temperatures will be between 25 and 28 degrees.
The national forecaster has issued a status yellow high-temperature warning for the entire country that is in place from 6am on Monday until 9pm on Friday.
"Temperatures will hit 27 degrees Celsius in places today and are expected to exceed it on subsequent days, this week."
Level Yellow High Temperature Alert
Valid from 06:00 hours Mon, 25-Jun-2018
Valid until 21:00 hours Fri, 29-Jun-2018June 25, 2018
Pat Clarke from Met Éireann says a warning like this is rare for Ireland.
"Basically, temperatures are going to continue to rise this week - around 27 degrees or so today and they will be in the high-20s in subsequent days," said Mr Clarke.
"It's a rolling kind of a warning and it will be updated every day. It's just to flag that it has been warm but it is going to get warmer."
Health Minister Simon Harris has urged the public to check in on elderly or isolated neighbours and make sure babies or children are not left alone in cars.
The Department of Agriculture has an orange fire warning in place and the HSE has issued essential heatwave advice as temperatures continue to soar.
If you are working outside in the sun, you need to wear sun-cream, take breaks and stay hydrated.
Pet owners are being advised to walk their dogs early or late in the day to avoid heatstroke, and use sunscreen on cats ears to avoid burning.
The DSPCA's Gillian Bird says if you have any doubt, check with your vet.
"We need to be conscious of things like how hot the pavements are going to get, especially if you've got tarmac - that can get very, very hot.
"So possibly walking your animals early in the morning or later in the evening [is best] and certainly not in the heat of the day.
"There's never any harm in putting your hand down on the ground to see how hot it is."
- Digital Desk