The IMPACT trade union has said that 800 social care workers are owed at least €10m by the Health Service Executive in unpaid premium payments.
National Secretary Louise O'Donnell said that from 2004 to 2013, the employees were entitled to "twilight" payments of time and a sixth for work between 6pm and midnight.
The entitlement was subsequently reduced to the period between 8pm and midnight, but staff were never paid the money, according to Ms O'Donnell.
However, she said staff were particularly irate that nurses and other staff working alongside them had received the payments.
Ms O’Donnell acknowledged that social care workers were now receiving the premium but said they are due arrears for the period from 2004 to 2013.
She said the bill was calculated on the basis of 1 hour and 20 minutes a week at €19 per hour.
Ms O'Donnell said a formula had been devised to address the issue for staff directly employed by the Health Service Executive.
However, she said staff were now balloting for industrial action at other voluntary organisations providing services.
Ms O'Donnell said staff were owed €2m at Dublin North HSE, as well as €7.5m at the Brothers of Charity and Ability West.
Staff in the west have backed industrial action by a margin of 98% and staff in the southeast are now preparing to ballot after conciliation talks broke down.
Ms O'Donnell said the union had taken legal advice, which said they had a "slam dunk" case and they had planned to get all staff to sue the HSE individually.
Social care workers provide child and disability services.
