One-in-five trains were at least five minutes behind schedule in November.
Iarnrod Eireann's punctuality index shows 80.55pc of DARTs were on time but the remainder -- 19.45pc -- were delayed.
It meant November was the worst month for the service.
Nearly every other period was above the 90pc mark, with March and April each hitting highs of 92.88pc.
Punctuality levels this year dipped below 90pc for the first time in October, when 88.13pc of trains were on time.
However, there then followed a significant fall in November and, with the weather worsening in December, more declines are expected. Iarnrod Eireann works to National Transport Authority (NTA) criteria, defining punctuality as less than five minutes late.
When it came to reliability -- whether a train operated or not -- the DART performed to near maximum levels this year. It broke the 99pc point every month and even hit 100pc in September.
The semi-State company said all the figures are independently verified by the NTA.
The punctuality data includes delays outside of Iarnrod Eireann's control, like trucks hitting bridges and extreme weather conditions such as snow or fog.
More than 97pc of commuter trains arriving between Heuston Station and the Greater Dublin Region were on time throughout the whole year.
Common causes of delays are signal faults on the line or, during periods of cold weather, malfunctions on the track.
However, Iarnrod Eireann has begun installing "points heaters" at key locations around its network, with high-priority locations in Dublin now equipped with the new technology. The equipment has been installed at Howth, Howth Junction, Clonsilla and Maynooth.
Points permit trains to move from one track to another and, at times of extremely cold weather, may freeze up and cause delays. The new heaters can be activated when temperatures drop below a certain level.
The company says it has an extreme weather plan, ensuring that if heavy snow occurs, every effort is made to keep services running.
"We know from 2010 that people rely on us more than ever at times of extreme winter weather."