SIPTU says Dublin Fire Brigade staff are at risk of burnout because of low staffing levels.
The union says the emergency response roster is running at 75 per cent of its recommended numbers.
It has also raised concerns that two extra ambulances, that were provided as part of the pandemic response, are no longer available.
The organisation says fire engines are unavailable on a "daily basis" because there isn't enough crew members to operate them.
SIPTU sector organiser Brendan O'Brien says the service must be properly resourced, despite the current economic challenges;
"We're well aware that the economy is struggling because of the Covid pandemic and we're all making sacrifices as part of the national effort but there are certain services we have to ensure are maintained at full capacity, including our emergency services."
