Firefighters and paramedics have joined forces with psychiatric nurses to form a new trade union for frontline staff in the public service.
More than 700 members of Dublin Fire Brigade have already signed up to join the alliance, headed by the Psychiatric Nurses Association (PNA).
Protecting premium pay and prevention of, and compensation for, injuries sustained in work will be among the issues tackled by the union leaders.
Des Kavanagh, general secretary of the unnamed group, revealed hundreds of fire and ambulance personnel have signed up, leaving the country's largest unions Siptu and Impact, and he said he expects to have more than 2,500 new members in the next six weeks.
"The huge transfer of members is being led by many who held leadership positions in the other unions," he added.
Mr Kavanagh said the PNA will give an independent voice to its firefighters and paramedics, whose sections will be represented by the Irish Fire and Emergency Services Association (IFESA) and the the National Ambulance Service Representative Association (NASRA).
John Kidd, former convenor for fire crews in Dublin, said the move reflects workers' dissatisfaction with the Croke Park deal.
"It is important to point out that our concerns are much deeper and can be traced back over many years when our reasonable claims were not processed, when grievances were not represented, when the aspirations of firefighters were repressed internally," he said.
"In establishing the IFESA we are joining our colleagues all over the world who independently represent the interests of firefighters and indeed the community we ... protect and represent."
Siptu general president Jack O'Connor maintained the numbers of those leaving the union had been greatly exaggerated, but admitted some were joining the PNA over their opposition to the Croke Park Agreement.