Last month was the second-driest September on record, according to data from the Armagh observatory.
The observatory said it recorded just 3.55mm of precipitation in the entire month - almost 20 times less than the average rainfall expected in September.
That means it is the driest September in the county for 120 years - since 1894, when just 2.7mm were recorded.
Met Éireann, which published its monthly weather bulletin today, said the month was generally the driest since 1986 in the south, southwest and west regions, with rainfall below 25% of the average in most places.
A spokesperson said the low rainfall totals were due to a period of absolute drought - defined as 15 days or more with less than 0.2mm each day. That happened from September 1 - 21 at Valentia Observatory, Roches Point, Newport, and Mace Head.
Valentia Observatory also recorded its sunniest September in 40 years, at 144% of the average - 166 hours of sunshine. Belmullet recorded its sunniest September day since 1969, when it recorded 12.3 hours of sunshine.
Shannon Airport, however, suffered from heavy showers on one day - September 5 - which was the third wettest September day on record, bringing its average rainfall close to the average.
Average temperatures were also above the expected average - by 1.5 degrees or more, with Shannon Airport recording 23.9 degrees as the monthly high.