Attempts will be made this week to resolve complaints by 51 immigrant workers who claim they are owed €500,000 in back pay.
SIPTU officials are seeking reimbursement of the wages for the Romanian carpenters, who, they claim, have been paid one-third of the agreed industry rate for the past five months.
The union's construction branch says that the men, who were hired by a Co Kildare sub-contractor, were paid around €5 per hour. The hourly agreed rate between unions and the Construction Industry Federation (CIF) for such work is €14.58.
Representatives from SIPTU's construction branch held an emergency meeting on Friday with members of the CIF and one of the major contractors whose sites the men have been working on.
A branch spokesman said that they presented documentation to the CIF - the management association for the construction industry - to allow it to investigate the alleged abuses. This included contracts issued to the workers and their time sheets.
The spokesman claimed that the sub-contractor had tried to take the men's passports from them last Thursday night after the union had been informed of the alleged underpayments.
The sub-contractor is not a member of the CIF, which has industrial agreements with trade unions on hourly rates for construction workers. While the Romanians were recruited by the sub-contractor, they were working on sites operated by CIF members.
SIPTU says that the main building contractors have a responsibility to ensure that all workers on their sites are paid the correct rate. According to the spokesman, 39 of the men are living in caravan accommodation in Co Kildare, while others are in houses.
A spokesman for the CIF said that it would begin today to "carry out a detailed investigation and try to have all the matters resolved as quickly as possible".
"There are agreed rates in the industry and all workers, whether they are foreign or Irish, should be paid them," the spokesman added.