Update 7pm: Tesco Ireland have called on workers to cancel their planned strike.
Mandate trade union announced an indefinite strike that will begin on Valentine's Day, as part of a row over contract conditions for long-term employees.
Tesco is calling on the union to agree to a Labour Court recommendation on pay, and says any employee who has their conditions changed will be compensated.
Christine Heffernan, Corporate Affairs Director with Tesco Ireland, says the conditions need to be updated to give the company flexibility.
“The terms and conditions of the pre-1996 contracts are nearly a quarter of a century old. They were agreed before our supermarkets opened late nights, weekends and before the rise of online shopping and, as a result, today we have too many colleagues working at quiet times of the week and not enough working during the busier times of the week to meet our customers’ demands.”
Earlier: Workers in nine Tesco stores are to go on all-out strike from St Valentine's Day.
The Mandate trade union said management were notified of the walk-out after members voted by a margin of 78% in favour of strike action.
The union represents 10,000 members in Tesco.
Tesco Ireland said: "Less than half of around 1,000 workers across 10 stores voted in favour of strike action and one store voted to reject industrial action."
The strike will continue indefinitely and could hit a further 15 stores when more ballots are completed, the union said.
The dispute centres on changes to employment contracts affecting about 250 workers who were hired by Tesco before 1996.
Mandate claimed that some workers will see their pay cut by up to 15% while they will be obliged to work more flexible hours.
John Douglas, the union's general secretary, said Tesco is the most profitable supermarket in Ireland and it plans to restart dividend payments to shareholders this year.
"Meanwhile, Tesco workers in Ireland who have worked with the company for more than 21 years, and are already classified as low-paid on slightly more than 14 euro per hour, are being told to accept imposed changes to their contracts or get out the door," he said.
Tesco said contract changes will affect less than 280 staff.
It said it has been through 12 months of negotiations and secured a Labour Court recommendation which cleared the way for changes to contracts signed before 1996 before late nights, Sunday opening and online shopping became the norm.
"We want to improve our customer service but these terms are no longer suitable as we have too many people working during the earlier quieter times of the week and this also creates unfairness between colleagues.
"We have always said that we will compensate colleagues for this change," Tesco said in a statement.
The strike action is taking place at Tesco stores in: