Staff at eight Tesco stores mounted pickets from 7am as members of the Mandate trade union commenced an indefinite strike in a dispute over contracts.
Tesco wants to move 250 staff recruited before 1996 onto less favourable contracts but Mandate insists the change would hit workers in terms of both pay and conditions.
Tesco says all its stores will remain open.
Last year, Tesco wanted around 1,000 staff recruited before 1996 to transfer to a less favourable contract which has applied to all staff hired since then.
Around 700 took redundancy but 250 staff continue to resist the move and have rejected a Labour Court recommendation on the row.
However, the union says 1,200 of the company's 14,500 employees were originally affected, but 900 have taken redundancy.
Mandate members are commencing an indefinite strike in eight stores: Baggot Street, Ballyfermot Road and Clearwater Finglas in Dublin, as well as stores in Tralee, Longford, Navan, Tullamore, and Bray.
Mandate says eight more stores will commence industrial action on Friday, with a further 22 stores to be balloted from Monday.
Tesco insists affected stores will remain open and has urged Mandate to accept the Labour Court recommendation.
It claimed a third of stores balloted had rejected industrial action, and that in stores that had been balloted, fewer than half of the employees had actually voted for strike action.
Tesco has also condemned Mandate's call on trade union members to "shop with their conscience", which could see customers shopping in non-unionised retailers.
Mandate General Secretary John Douglas said members rejected the Labour Court recommendation on the basis that it still imposed cuts on workers, who are already on meager wages.
Speaking on RTÉ's Today with Sean O’Rourke, Mr Douglas said the economic facts do not stack up and 250 workers across 50 stores will not have an impact on the bottom line for Tesco International.
He added that there are 3,000 workers, many of whom have fixed hours, who are paid more than the workers in dispute over contracts because pay increases have not been implemented for those in dispute.