The Financial Services Union said the time has come for Finance Minister Paschal Donohoe to raise concerns about the potential sale of the Ulster Bank loan book with UK chancellor Rishi Sunak.
Media reports have suggested that Ulster Bank's parent group NatWest is in talks with Cerberus to buy its loan book, which is estimated at €20.5 billion.
Cerberus was a major buyer of Irish distressed debt after the financial crisis.
"This speculation has been running on for a couple of months and a clear statement of intent from the Minister for Finance is required, stating that he is willing to contact the chancellor and outline the Government's opposition to any vulture fund buying any part of Ulster Bank," the general secretary of the Financial Services Union, John O'Connell, said.
Mr O'Connell said the UK government has a considerable ownership stake in the bank.
He said the Government here should be using every lever at its disposal to ensure that jobs are protected, branches remain open, and there is sufficient competition in the market place.
"I ask the Minister for Finance to contact his counterpart in the UK government and to outline in clear terms the Irish Government's opposition to NatWest selling any part of the Ulster Bank loan book to a vulture fund," Mr O'Connell said.
"NatWest needs to end the speculation and commit to the future of Ulster Bank," he added.