Tesco stores in Sligo and Carrick-on-Shannon will see strike action by Mandate members in the run up to Christmas in a dispute over issues including canteen facilities.
Strikes will take place at the Sligo store tomorrow and on 14 December - with a further stoppage at the Carrick-on-Shannon store, Co Leitrim, on Saturday 22 December.
However, Tesco has assured customers that the Sligo store will remain open tomorrow from 8am to 10pm.
Both Tesco and Mandate have accused one another of breaching agreed disputes procedures.
In a message on the Mandate website, Assistant General Secretary Gerry Light accused Tesco Ireland of refusing to deal reasonably with a series of outstanding local issues including canteen facilities.
However, he said problems extend beyond Sligo and Carrick-on-Shannon, and claimed that over the past three years, Tesco's long term strategy involved trying to get rid of Mandate from its Irish business.
He described the management position as an outright attack on workers' established terms and conditions of employment with the ultimate objective of destroying their union and weakening their power at work.
He urged workers in other Tesco stores to support their colleagues in Sligo and Carrick-on-Shannon.
Mr Light said: "Tesco no longer collectively negotiates your core terms and conditions such as pay, weekly hours and rosters, instead management now decides how much you get and when you get it."
Tesco responded by saying that the terms of its collective agreement did not give rise to an automatic pay increase each year.
It said that rather than subject staff to a lengthy process of negotiation, the company had made a 2% pay award and a 2% discretionary bonus to workers.
Tesco said this put their workforce at the higher end of the scale in terms of pay awards being made nationally.
Mandate also raises the issue of 180 staff recruited prior to 1996, who have not received a pay rise for some years because they have refused to change their contract to work less favorable hours operated by their post-1996 colleagues.
Tesco noted that the company had accepted a Labour Court recommendation on the issue - and said the pre-1996 staff would receive the benefit of a pay award when they move to a new contract.
Mr Light claims that Tesco no longer fulfills its obligation to attend at the Workplace Relations Commission or the Labour Court, instead attending "when it suits them" - and would not allow union officials to represent members or permit union correspondence to be placed on instore notice boards.
In a statement, Tesco said that the dispute in Sligo and Carrick-on-Shannon involved local issues and did not warrant strike action.
It said the company continues to offer the best pay and benefits in the sector, and remains the only food retailer to recognise Mandate.
Tesco accused the union of operating outside normal industrial relations procedures by refusing to engage at the WRC.
It rejected Mandate claims that the company had itself refused to engage at the WRC until recent days.
Tesco said it fully respected the right of workers to choose to be represented by a union - but that Mandate had chosen to move away from their obligations under collective agreements.
It said it was engaged in a €3m project to enhance canteen facilities in its stores.