High temperature warnings are in place for eight counties with highs of up to 31 degrees forecast.
Met Éireann has issued a status yellow alert for Clare, Kerry, Limerick, Tipperary, Galway, Kilkenny, Laois and Offaly until 6pm on Wednesday.
The forecaster is warning there's a risk of heat stress, forest fires, and water safety issues due to increased use of lakes and beaches.
A new record was set for the month of May when a new high of 30.5 degrees was recorded at Shannon Airport on Tuesday.
GALWAY CITY, IRELAND Salthill promenade on the beach in Galway 5th May, 2018Andrew Doran Sherlock, from Met Eireann, says we could also be facing some very severe thunderstorms with conditions becoming more unsettled in the coming days.
"There is likely to be thunderstorms pushing up from the south this afternoon.
"Basically, because we've had so much daytime heating, there's a lot of available energy to fuel these if they kick off.
"So they could actually be quite severe in places. There is the potential of warnings being required today for that."

"The temperatures are declining and it's looking like it'll turn more unsettled for the long weekend.
"For Thursday, there'll be a fair amount of cloud, mostly dry weather, but there will be the odd spot of light rain and drizzle.
"There'll be a good deal of dry weather, on Friday with some isolated showers.
"On Saturday, a band of rain is expected to move eastwards over the country, and there could be some heavy showers in places, while Sunday is due to be the best day of the long weekend".