A confidential pay deal seen by RTÉ indicates that SIPTU had negotiated a pay reduction for waste disposal workers transferring from a rival to Panda.
Panda secured the deal in relation to 18 workers transferring from a rival firm.
The move was part of Panda's takeover of the green bin service for Dún Laoghaire Rathdown Council in mid-2011.
The revelation comes amid a stand-off between Greyhound management and refuse collectors which is entering its sixth week.
Greyhound is refusing to reinstate around 80 workers to their former salaries.
The company has proposed pay cuts of just over 30% for drivers and operatives.
However, union sources said that the pay agreement in 2011 was not directly comparable to the current Greyhound stand-off.
The union says that Panda workers had been paid a lump sum for their lost earnings.
No such offer has been made as part of the Greyhound dispute.
In a letter dated 7 June 2011, SIPTU wrote to Dún Laoghaire Rathdown council's then manager, Owen Keegan.
The letter informed the council that a deal had been reached between the union and Panda.
It noted that it was a significant step forward in relations between SIPTU and Panda.
Paul Smyth, a former SIPTU official who played a key role in the 2011 agreement, confirmed that the staff transferring into Panda had accepted significant cuts in pay.
However, he said the situation was different to the Greyhound stand-off.
He said the Panda deal involved a small number of staff transferring into a different company.
Panda was a company with a much larger number of employees who were already on lower pay rates, he said.