Siptu has served Whelan Frozen Foods with a notice of industrial action in a dispute over redundancies and the transfer of undertakings.
Whelans has supplied Dunnes Stores with frozen foods and textiles for over 25 years, but the contract is due to terminate on February 28th.
The company - which employs 460 staff in Dublin - faces an uncertain future as Dunnes Stores is its sole customer.
Siptu said: "Last December, management at Whelan's gave the union a commitment that it would clarify the company's future for the workers, but to date no such clarification has been received and management has failed to engage with the union on the issue."
Siptu branch organiser John Dunne said: "Some workers may be made redundant at the end of February. But so far there is no agreement on how many or under what severance terms. Some may remain in employment at their existing location but with new contracts.
"But so far there is no agreement on how many and under what terms and conditions," he added.
The union industrial action - up to and including strike action - will take place from midnight on January 25th unless talks on these issues get under way.
The move is likely to affect supplies to Dunnes Stores in the frozen food and textile goods area.