The brigade has shelled out €3.5m in out-of-hours payments so far this year, despite a cost-cutting drive.
The figure is €400,000 more than last year and €410,000 -- or 13.3pc -- more than the tab for 2010.
Dublin City Council, which funds the service in conjunction with the other three local authorities in the capital, said minimum staff numbers are needed every day.
However, the number of vacancies has almost doubled in two years.
"In order to provide fire, emergency rescue and ambulance service, minimum manning levels are required on a daily basis," the council said.
When operational shortages occur, they are replaced with overtime, it added.
"The number of operational vacancies has now risen to 82, from 44 at the end of 2010," the local authority said.
The information was provided to Fine Gael's Kieran Dennison, who has questioned the amount of money Fingal County Council is paying for the service.
In order to provide emergency cover on a 24/7 basis, staff work day and night shifts, including every second weekend. Their pay has been set nationally and is divided between basic wages (67pc) and allowances (33pc), the council said.
The allowances are paid for working shifts, public holidays and to cover rent.
Some 882 staff are currently employed by the fire brigade, costing €61.67m last year in wages.
The figure was a reduction on the €62.64m and €63.58m payroll bills in 2011 and 2010, respectively.
Council chiefs have already begun negotiations with fire Brigade representatives over further budget cuts of up to €3m.
If the savings are achieved, it will mark a €6.5m reduction in the money spent on the operation.