Mandate has accused Dunnes Stores of targeting staff who participated in a countrywide strike last Thursday.
In a statement this afternoon, the union said it has had reports from members all over the country who have experienced dismissals, cuts to hours, changes in roles and changes in shift patterns after the strike.
Mandate Assistant General Secretary Gerry Light said: "This behaviour by management in Dunnes Stores is deplorable and despicable and must be condemned by everybody."
He said the actions by Dunnes over the last five days has only strengthened the resolve of Dunnes workers who are now more determined than ever to achieve their objectives of decent work.
Thousands of staff at Dunnes Stores staged the one-day in a dispute over low-hour contracts, job and income security and the right to trade union representation.
The industrial action affected 109 stores countrywide.
Mandate said more than 6,000 of the union's members at Dunnes Stores took part in the strike.
Speaking on RTÉ's Drivetime, Mr Light said that following the behaviour by management he expects more disruptive action by union members "in the very near future".