The current record temperature for September was reported in Clongowes
Tomorrow's temperature could break all records for September, Met Éireann has said.
A Status Yellow high temperature warning for Ireland is in effect with a peak of 28C predicted for today, but temperatures could surpass this in some places, according to meteorologist Gerry Murphy.
The current record temperature for September is 29.1C was reported in Clongowes, Co Kildare in September 2009.
Speaking on Thursday morning, Mr Murphy said Met Éireann expects tomorrow will be the hottest day of the current sunny spell, when the existing temperature record for September could be broken.
Met Éireann has issued a thunderstorm warning for six counties in the midlands and north-east.
Cavan, Monaghan, Longford, Louth, Meath, Offaly and Westmeath are under the alert until 6 o'clock this evening.
Meanwhile, a separate warning for the entire country for high temperatures remains in place until Saturday.
Emer Flood from Met Éireann outlines where the hottest temperatures are expected: