Offaly's Cathal Nolan says Leinster will bear the brunt.
Today could be the hottest day of the year so far.
Met Éireann is forecasting temperatures of up to 28 degrees in Leinster and Ulster.
Severe downpours, hail and spot flooding could follow, though.
A yellow thunderstorm warning will be in place throughout Ireland from 4 o'clock this afternoon until 9 o'clock tomorrow morning.
Offaly's Cathal Nolan, from Ireland's Weather Channel, says Leinster will bear the brunt of the storm: