SIPTU members working in Mental health Services in Ballinasloe will ballot for industrial action this week.
The row workers centres on assistants and care workers in St Brigid's Mental Health Intellectual Disability services.
The union is accusing the HSE of failing to consult with them about the outsourcing of their work under the reconfiguration to a social model of care.
The assistants and care workers have been working with service users who live in group homes, but they are not being considered for the same work when the service users transition to Independent Living.
SIPTU organiser Yvonne McGrath says the situation has left the union members with no choice but to consider industrial action.