The country's largest union SIPTU has described the British government as "destructive" and" pompous", and accused it of "mind-blowing indifference" to the peace process.
Speaking at SIPTU's Biennial Conference in Cork, General Secretary Joe O'Flynn warned that Brexit will be catastrophic for tens of thousands of workers and their families on the island of Ireland.
He said many will pay a heavy price with the loss of their jobs and the livelihoods, with rural communities north and south devastated by the economic fallout , adding that some have already been affected.
He said that despite SIPTU's attempts to institute measures to offset negative impacts, it was almost impossible to do so when no one knew the direction, or indeed the ultimate destination of what he called the "destructive and pompous UK government".
He also derided what he called the "debacle" of Brexit and the "mind-blowing indifference to our fragile peace process which we have nurtured for the past 21 years".
Mr O'Flynn said that while the economic fallout of a 'no-deal' Brexit would be highly negative, "...worse still, will be the political crisis which may see a return to a hard border and a threat to the Good Friday Agreement".
He voiced fears of a return to unrest if the extremists on both sides moved to exploit the situation for their own agenda, adding: "... and they will".
He stated: "We have seen evidence of it already in recent times and, I believe, all of us on this island of ours will pay a very heavy price for the indulgence of the Tories and their populist politics".
He went on: "It is clear that they have never once stopped to consider the implications of their actions on the lives of our people, north or south".
Mr O'Flynn also attacked what he called "the horrible and hate-filled politics of the far right" which had gripped many nations and had propelled the planet to one of its most fragile conditions in terms of issues including wars and unrest, along with displacement on a scale never witnessed before.
He cited economic uncertainty, trade wars and tariffs at will, with no thought to the consequences for the workers affected.
He also alluded to "excessive obscene profits and influence by a very powerful minority - the 1%".
Mr O'Flynn also warned of the imminent threat to the planet from rapid climate change, and called for a just transition arrangement for workers dependent on the production of fossil fuels to protect communities particularly in rural Ireland.
He also demanded better collective bargaining rights for workers, and highlighted SIPTU's work to enhance pay and conditions of employment including pensions, health and safety and respect at work.