Dublin Fire Brigade was understaffed for more than two-thirds of the days in September, according to new figures released to Newstalk.
One particular day saw the service 45 members short of what is required.
The Freedom of Information figures show that Dublin Fire Brigade was understaffed for 23 out of the 30 days in September.
The minimum number of staff needed for daytime shifts is 180. That includes all frontline staff and staff in the control centre.
The 10th of the month saw the service stretched the most - with six fire appliances off the road and 135 staff out of the 180 on duty.
Reliance on overtime
SIPTU said understaffing is causing members to be fatigued.
The union's Industrial Official Geoff McEvoy said it comes as no surprise.
"The figures revealed by the FOI request just confirm what firefighter paramedics have been experiencing for a considerable number of years now," he said.
"The impact of it, in terms of taking appliances off the road, is obviously having an impact on the health and safety of firefighters and the general public.
"It's creating fatigue and burnout, because there is a huge reliance on overtime," he added.
Staffing issues also saw ambulances taken off the road.
There were two ambulances each on the 7th and 27th without staff.
Dublin Fire Brigade says it takes the health and safety of its crews and the public very seriously.
It also says 80 recruits have passed out of their training centre since January of this year, with a further 30 expecting to become operational in the coming weeks.
They are currently advertising a new recruitment drive for firefighters.