SIPTU has served a 3.5% pay claim on the Health Service Executive on behalf of 34,000 health workers.
Paul Bell of SIPTU said the increase was the first tranche of the 6% increase agreed by the Social Partners last September.
He said that a moratorium on public sector pay increases had expired and that the money was now due to his members.
The grades affected range from porters and catering staff to radiographers and nurses.
The HSE said that pay within the organisation 'like all public service organisations, is a matter for the Department of Finance'.
The move will increase pressure on the Government, which is currently exploring ways to cut the €20bn public sector pay bill.
There has been speculation that the Government may be considering an across-the-board pay cut for State employees, over and above the pension levy that was introduced earlier this year.
The public service is already subject to a pay freeze as well as a recruitment embargo.
Incentivised career break and early retirement schemes have also been introduced to try to reduce numbers on the State payroll.
Earlier, Tánaiste Mary Coughlan said unions should sit around the negotiating table, rather than engage in strike action.
Responding to last night's turnout of over 1,000 people at the first meeting of a new alliance to fight pay cuts, Mary Coughlan said the McCarthy report was only part of the discussions and preparations for the Budget.
Speaking in Dublin, Ms Coughlan said no-one wanted to see a reduction in frontline services, but she said savings still needed to be found.
'We do have to find our savings and that is on-going,' she said.
Ms Coughlan acknowledged that there was a considerable amount of concern from those in frontline services 'who don't want to see a diminution of those services'.
'None of us want to see that happen,' the Tánaiste added.
