The Communications Workers' Union has said it will ballot its members at HCL Technologies, a key service provider to Eir, for industrial action.
This is in response to the "derisory" redundancy packages offered to the union by company management at meetings yesterday.
Telecoms company Eir last week said it was bringing up to 950 customer service jobs that had been outsourced, back in-house again.
Up to this, the company's contact centres had been managed and provided by staff of Indian technology firm, HCL Technologies.
CWU General Secretary Steve Fitzpatrick said that HCL management had presented redundancy packages for up to 650 workers at the company that are impacted by last week's announcements.
"The packages are derisory and unacceptable. We are now proceeding to ballot for industrial action at HCL, up to and including strike," Mr Fitzpatrick said.
Eir said last week that 300 staff working in HCL centres in Cork, Limerick and Citywest will move over to the former state-owned telco directly under the Transfer of Undertakings Directive.
This governs how staff should be treated when a company's business is transferred to another.
However, the HCL office at Telephone House on Marlborough Street in Dublin city centre will shut in the first three months of next year.
650 staff working there will also have the option of moving to other roles in Eir, but these jobs will be based in Cork, Limerick or Sligo.
Alternatively they can take redundancy from HCL.