Representatives of thousands of frontline public servants, including fire fighters and ambulance paramedics, are leaving SIPTU to form a new trade union in alliance with the Psychiatric Nurses' Association.
PNA General Secretary Des Kavanagh said the primary aim of the new organisation would be to protect frontline services to the public, which he said were currently under immense threat.
Mr Kavanagh was speaking at the launch of the union, whose name has not yet been decided.
He said that they would also be fighting to protect the pay and conditions of frontline staff, especially premium payments and allowances.
Mr Kavanagh said that he anticipated that the new union could attract up to 2,500 members over the next six weeks.
Derek O'Rourke, who represents ambulance personnel, criticised SIPTU for showing a lack of leadership and failing to take on the Government.
SIPTU President Jack O'Connor said it had been aware that some members had been unhappy with the Croke Park Agreement.
Mr O'Connor said: 'We respect the rights of people who take a different view. However, the issue was decided democratically by a secret ballot vote among our members who are employed in the public service.
'This resulted in the endorsement by SIPTU of the proposals. They were subsequently endorsed by a majority of the public service unions.'