For some parts of the country, it was the coldest September in 21 years or more, according to Met Eireann.
All monthly mean air temperatures were below their long-term average (LTA) with stations reporting deviations from normal of -0.4 degrees Celsius or more, it said.
Seven stations reported the coldest September in 21 years or more, with mean temperatures varying from 11.6 degrees at Mullingar (-1.0 from its LTA), 11.6 degrees at Ballyhaise, Co. Cavan (-1.1 from its LTA) to 13.0 degrees (-1.2 from its LTA) at Shannon Airport.
Most of the highest temperatures were recorded on the September 8, with the majority of maximum temperatures recording their lowest maximums for seven years or greater.
Eight stations reported ground frost, with Oak Park, Co. Carlow and Mountdillon, Co. Roscommon reporting four days each in the month.
The majority of monthly rainfall stations had below average rainfall for September, Met Eireann said.
The percentage of LTA values were lowest at Malin Head (31% of its LTA) with a monthly rainfall total of 29.7mm.
The percentage of LTA values were highest in Mace Head, Co. Galway with 144%. The station reported 161.3mm of rainfall for the month, with over a third (58.4 mm) falling on the September 11.
Newport, Co. Mayo also had its wettest day on the September 11, recording a daily rainfall total of 79.4mm, its wettest September day since records began 55 years ago.
A couple of stations in the southwest also reported this day as their wettest September day in over 22 years, Met Eireann said.
Malin Head had its sunniest September since 2002 and monthly sunshine totals ranged from 69.1 hours at Knock Airport to 136.4 hours at Dublin Airport, it said.
The most sunshine in one day was 12.6 hours reported at Belmullet, Co. Mayo on the September 8. while Malin Head had its sunniest September in 13 years with a monthly total of 129.2 hours, Met Eireann said.
The number of dull days (days with less than 0.5 hours of sun) ranged from three at Valentia Observatory to eight at Knock and Shannon Airports.