Brexit could spark a "race to the bottom" for workers' rights, one of the country's most senior trade unionists warned.
Patricia King, general secretary of the Irish Congress of Trade Unions (ICTU), said it could spark a push by companies here to cut wages and labour costs, placing huge pressure on workers.
"In order to bring down the price of trading, companies will...believe that they are forced to pay less, or bring down their labour costs," Ms King told a UK House of Lords inquiry examining the impact of Brexit on UK-Irish relations.
"That will put huge pressure here, and indeed in Northern Ireland, on workers."
She said small and medium sized companies will be affected in particular by the potential pressure to cut costs.
"There would be a strong push.
"Even something like the minimum wage, for example, whatever way the UK handles that, all of that will have an effect," Ms King added.
"And we'll be told here, 'if you want to be competitive, this is how you do it'."
Ms King said Foreign Affairs Minister Charlie Flanagan has already told Congress that this is a big issue.
"I have already heard companies on the airwaves saying that the only way that they could foresee continuing this trade, and to survive, is to deal with labour costs. They've already started this," Ms King said.
The committee was taking evidence in Dublin yesterday, as it continues to examine the impact of Brexit.
John McGrane, director general of the British-Irish Chamber of Commerce, said that it was unrealistic to think there wouldn't be some form of a Border on the island after Britain leaves the EU.
"We don't know of any technology that is so advanced that it can tell us the contents of a vehicle without the driver having to declare in a quite traditional way," Mr McGrane said.
"So there are a lot of fanciful ideas out there that say nothing much is going to change, the Border issue being just one of them."
Later, Kevin Sherry, Enterprise Ireland's executive director for global business development, told an Oireachtas Committee that an extra €3m provided in the Budget will see an additional 50 staff for the agency, IDA and the Department of Jobs.