Some of the former workers employed on the ESB's €3 billion networks renewal programme were paid only €3.21 an hour, according to the Technical Engineering and Electrical Union (TEEU).
The Serbian workers were employed by Energoprojekt Oprema, a Belgrade-based company, sub-contracted to Laing O'Rourke, the main contractor on the ESB project.
The going rate for the job in Ireland was €18.97 an hour at the time of their employment, under the industry's Registered Employment Agreement.
An investigation into payment at the company was carried out seven weeks ago by the Labour Inspectorate when it was reported that some Serbian workers on the project were earning €4.96 an hour.
However, the inspectorate has not yet submitted its report to the Labour Relations Commission and no action has been taken in relation to paying arrears to workers.
The TEEU obtained legal opinion on the wages slips and contracts provided to seven men who were paid €3.21 an hour and were told that the contracts breached seven separate pieces of Irish employment legislation.
Some 95 workers were registered as employed in Ireland by the company, but the TEEU maintains that up to 200 Serbians may have been employed here and they may be owed up to €4 million in arrears.
They also said that staff were never given PPS numbers and had their passports confiscated when they arrived in Ireland.
Energoprojekt Oprema was unavailable for comment yesterday.
In a statement originally issued in March, Laing O'Rourke said it understood that the workers had always been paid the correct pay rates.
"The hourly rates of pay have always been in the range of €17-€19 per hour paid by EnergoProjekt to its operatives," the statement said.
"In addition, the Labour Inspectorate has conducted an audit of pay and conditions for Energo operatives and no issue has arisen from these inspections."
