Thunder and lightning have hit many parts of the country as we begin to feel the effects of Met Éireann's status orange warning.
The alert took effect at 3pm on Sunday and will continue through until 9am on Monday for parts of the country, while Munster and southern Leinster will remain covered until 4pm.
The warning is expected to bring heavy showers of rain and high temperatures to many parts as the existing yellow high-temperature warning remains in effect until 6am.
It continues to cover areas of Munster, Leinster, Cavan, Monaghan, Galway and Roscommon bringing highs of 27C to 30C.
Heavy and thundery showers have already begun with downpours and spot flooding in parts of the country.
The forecaster says some places will remain dry, but "hit and miss" thunderstorm activity with hail and heavy downpours could lead to spot flooding in some areas.
Met Eireann's Andrew Doran Sherlock says the thunderstorms will hit quite sporadically so not everywhere will get affected.
"Every area is at risk, but not everywhere will see it because they can be quite localised developments," he said.
"One kilometer difference could dictate whether you're experiencing the heavy thunderstorm or you're not, but all areas are at risk."
People are being reminded to be careful with their use of water during the heatwave.
Irish Water is working to replenish supplies to customers in Wexford, Kerry, Donegal and Mayo.
A number of households in these counties have seen their water pressure reduced in recent days and nighttime water restrictions introduced, in an effort to avoid drought over the heatwave.
The high temperatures are believed to be connected to burst pipes in many plants, leading to interruptions in supply.
Firefighters in Carlow have brought a massive wildfire on Mount Leinster under control as fire brigades across the country responded to dozens of smaller gorse and wildland fires.
Dozens of firefighters in Carlow, assisted by a firefighting helicopter, spent hours battling a large gorse fire close to the Nine Stones area on Mount Leinster along the Blackstairs Mountains.
The fast-moving gorse fire was visible across counties Carlow, Wexford, and Kilkenny.