The sale of Irish Biscuits would have "devastating consequences" for workers and their families, SIPTU said today.
The union was responding to reports that the firm's parent company, Danone, may sell off the Dublin operation, threatening 435 jobs.
Danone is understood to be considering the sale as part of a strategic review of operations in all markets.
Irish Biscuits is best known in Ireland as the manufacturer of the Jacob's brand, which covers well-known products such as Jacob's Cream Crackers, Kimberley, Mikado and Coconut Creams.
SIPTU's Food Branch Official, Mr Brendan Carr, said: "The community in Tallaght has not yet recovered from the loss of 800 jobs following the closure of the Packard Electric plant and the loss of another 400 jobs does not bear thinking about."
Mr Carr said: "We have just concluded a restructuring deal for Irish Biscuits and there was no hint of selling the plant.In fact we secured a commitment from the management as to its future."
Irish Biscuits was formed in the 1960s from the merger of Jacob's and Boland's Biscuits. It has an annual turnover of some €100 million, but this is tiny in the context of the €13 billion in sales recorded by Paris-based Danone last year.