SIPTU has said its members employed as retained firefighters are "outraged" by the response of Minister for Housing, Local Government and Heritage Darragh O’Brien to their demands for improved pay and conditions.
A second day of rolling strike action by retained firefighters is under way.
The work stoppages are leading to the closure of up to 50% of fire stations in the service.
SIPTU said the action will be escalated to an all-out strike next week if the dispute is not resolved.
In a statement to the Dáil yesterday evening, Minister O'Brien said he was committed to ensuring that the retainer payment received by part-time firefighters will be positively dealt with in the upcoming public sector pay talks.
"I will be a strong advocate for and champion of firefighters on the retainer issue in the upcoming collective pay talks," Mr O'Brien said.
SIPTU described the response as nowhere near good enough and said that it has made resolution of the dispute more difficult.
"As our members take to the picket line and protest for a second day, they are making clear their universally angry reaction to the statement on the dispute that Mr O'Brien made to the Dáil yesterday," said SIPTU Public Administration and Community Division Organiser Karan O Loughlin.
"They now hear the minister saying that while he supports their cause, he can do nothing about it in advance of new public service pay talks," she added.
Members of the retained service are part-time firefighters who are paid an annual retainer to be available for call-outs.
SIPTU represents around 2,000 retained firefighters across 200 stations around the country.
The union said there is a recruitment and retention crisis in the service meaning that many firefighters are unable to take their leave entitlements due to staff shortages.
They have also seen their incomes drastically reduced due to reductions in call outs over the last number of years.
