A warning for forest fires has been upgraded to red due to the dry conditions and high temperatures.
It is the highest warning level and is rarely issued.
The Department of Agriculture says the alert will remain in place until 12pm on Friday - with people advised not to light barbeques outside designated areas and to take care when disposing of cigarettes.
The Department says any spark could give rise to the spread of a rapid and unpredictable wildfire.
The risk is particularly high in areas where there are dead grasses and shrubs like gorse and heather.
They are also warning that upland fires which usually cover large areas pose extreme difficulties and could develop into a major emergency.
It comes as Coillte staff have been fighting a forest fire at Barnaslingan Wood for the past nine days, along with Dublin Fire Brigade.
Teams have been working around the clock to keep the fire contained, but rain is needed to dampen the fire and cool off the embers in the ground.
Coillte has issued the following advice to prevent forest fires:
Irish Water says restrictions will become unavoidable if the heatwave continues.
The utility says demand levels for water in the Greater Dublin Area rose again overnight with some areas in Athlone, Kilkenny, Longford and north Dublin already experiencing outages.
A yellow temperature warning remains in place - with highs of 30 degrees expected this week.
Kate Gannon from Irish Water is appealing to people to conserve supplies.
"It's up to each individual to be mindful of their water usage at a time like this," said Ms Gannon.
"I appreciate it's lovely weather and we don't get it all that often and people are outside and they're enjoying themselves.
"But really every choice that an individual makes impacts not only on themselves but on their neighbours.
"So think of your neighbours that are up at the top of the hill or at the edge of the network."
Temperatures are set to rise even further today, as the heatwave continues.
Met Eireann is predicting top temperatures of 28 degrees later, but they are set to increase further later in the week.
A status yellow high-temperature warning remains in place until 9pm on Friday.
Wednesday could see temperatures reach 30 degrees as it continues to stay dry and sunny.
The hot and dry spell has put pressure on water supplies.
Irish Water is monitoring reservoir levels at a number of locations.
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
Kate Gannon of Irish Water says the public can help by taking a few simple steps.
"Don't use hoses to water your garden. If you are going to water things, water them late in the evening.
"Avoid washing carse, avoid power hosing. If you are going to have a paddling pool for the kids, please keep it as shallow as possible.
"And also consider taking short four to five-minute showers as opposed to taking baths."