State-funded bodies that provide services in the health, education and community sectors could face industrial action if they fail to restore pay cuts imposed during the economic crisis.
The warning has come at Fórsa's annual conference in Killarney.
The dispute centres on around 12,000 workers in so-called "Section 39" bodies, which rely on what was described as a "hotch-potch" of funding from agencies, including Government departments, the Health Service Executive, Tusla and Pobal.
While staff in such bodies are not technically public servants, many had their pay cut in line with public servants during the crisis.
However, despite the fact that the Government is now restoring pay to its own public service employees, many Section 39 workers have seen no such restoration.
Section 39 agencies argue that while they would like to restore or improve pay and conditions, their funding allocations have not been restored to permit them to do so, but unions dispute that all bodies are unable to pay.
SIPTU suspended threatened industrial action earlier this year to permit a review of pay and finances at a sample of Section 39 agencies, which was completed this week.
The review found that, on average, Section 39 workers had seen salary reductions of 4.66%, but that agencies varied in the extent of both salary cuts and initial restoration.
It also found that applying full restoration to all 302 Section 39 bodies would cost almost €68m.
Forsa official Eamon Donnelly said there was no better campaign than to try to restore pay to a group of people who had their pay reduced beasue they looked like public servants, but when pay restoration came back, did not have it restored becasue they were not public servants.
He said that Forsa and SIPTU had started with a somewhat different approach, but were now acting in a co-ordinated way.
Mr Donnelly warned that if the union now had to enter into a phase of industrial action, they would be targeting agencies that they believe have funding streams to be able to pay, but are hiding behind claims they are waiting for someone to write a cheque.
Delegate Imelda Reidy said that Section 39 workers were looking for "basic stuff" such as paid maternity leave, increments, and pensions that other people took for granted.
She said these staff were providing services to the most marginalised communities in Ireland, but their pay was being determined by the "whim" of their funders.
Kathleen O'Doherty of the School Secretaries Branch said that many school secretaries were in a similar position, working alongside colleagues funded by the Department of Education on better pay and conditions.
Delegates passed two motions calling for the employment terms of Section 39 workers to be brought into line with public servants.
SIPTU Health Divison Organiser Paul Bell welcomed the Forsa warning on industrial action, but reiterated his union's demand that Section 39 workers must see pay restoration commence before the end of this year.
Affected unions will meet next week to discuss their reaction to the HSE review.
Staff recruitment and retention problems across the public service
There are more severe difficulties in recruiting and retaining physiotherapists, occupational and speech therapists than there are in nursing, according to Fórsa.
The Public Service Pay Commission (PSPC) is currently examining areas where staff shortages have been reported, including for nurses, doctors and the Air Corps, with its report due next month.
However, Mr Donnelly said no public service group had a monopoly on staff retention difficulties.
He cited last year's PSPC report, which found turnover rates of 8% for health service therapy grades, and 8.8% for other health professions.
He told delegates that compared with 8.8% for hospital consultants, 3.3% for public health nurses, 3.4% for nurse specialists, 5.7% for nurse managers, and 7.7% for staff nurses.
Mr Donnelly urged the Government to be "even-handed" in its response to the current PSPC examination of the problem, adding that the union would not accept a situation where any profession or grade was treated more favourably than another in the Government's response to the commission's findings.
Fórsa official Andy Pike also noted that uncompetitive entry-level pay in parts of the civil service had also left state bodies struggling to recruit in grades including cleaners, solicitors, meteorologists, radio officers, agricultural officers, valuers, Oireachtas researchers and translation staff.
He called for civil service management to be given the flexibility to pay staff above the usual entry rates, where necessary, in order to attract suitably qualified staff.