Staff at the Fás Training Centre in Tallaght in Dublin are holding a 24-hour work stoppage in a row about overtime.
Siptu, which represents the six workers, claims its members are owed more than €13,000 in overtime pay under the existing local agreement.
Local branch secretary Brendan O'Brien claimed Fás management was withholding pay to the workers in an attempt to force agreement on new work practices.
He said a compromise proposal had been offered by Siptu last night but was rejected by management and this had resulted in a work stoppage and a picket being place at the west Dublin centre.
Fás, however, says the overtime money owed will be paid to staff as part of an new agreement on pay and conditions which have yet to be agreed.
A Fás spokesman said a deal with Siptu relating to overtime at Tallaght had been agreed at the Labour Relations Commission last November but this was later rejected by the union's members.
He said Fás regretted that such action was taking place over what is a localised dispute but he said the management remains available for talks.