Concerns are being raised about the lack of firefighters in the city.
SIPTU says there weren't enough staff to field a full fleet of fire engines on Wednesday.
Six fire engines were idle on Tuesday night, while five weren't running on Wednesday.
It says the service needs 36 officers and 134 firefighters at full strength.
But just 19 officers and 121 firefighters were available.
The union says the service is suffering from a serious manpower shortage.
It is calling for a new recruitment drive to take place.