Trade Commissioner Phil Hogan has compared tensions between the UK and EU ahead of talks to "argy-bargy" in the Croke Park tunnel before a match.
He raised concerns at the prospect of a hard Brexit and warned that if the UK wants to be taken seriously in forging global trade deals, "it has to honour its commitment to us".
Mr Hogan was speaking yesterday at the American Chamber of Commerce Ireland Transatlantic Conference in GAA headquarters, where he said better EU/US trade relations were a priority for him.
On Brexit, he referred to comments by the UK's negotiator David Frost last month suggesting Britain would reject "level playing field" conditions that the EU has been insisting on in a free-trade deal.
Mr Hogan said the "mood music" between the two sides had improved, but added that it was "difficult to take comfort" from the UK rhetoric.
He said that as with Croke Park, "maybe it's a bit or argy-bargy in the tunnel you can't do anything about".
"It's an absolute priority for us that the Withdrawal Agreement signed last October is implemented in both spirit and letter, including the commitment to no hard border on the island of Ireland," he said,
He added that the UK "was quite explicit that it wanted to avoid a crash-out Brexit".
Mr Hogan quoted what the UK agreed to as "the government's ambition is to conclude an ambitious, broad, deep and flexible partnership across trade and economic co-operation with the EU, with a free-trade agreement at its core".
"Mr Frost's recent speech seems to confirm that the UK will want to renege on these commitments and is pursuing a hard Brexit," he added.
"Perhaps this is part of the opening skirmishes that you have in any particular match."
He urged the business people present to use any influence they have in the UK "to make their voices heard, because a no-deal Brexit will bring nothing but pain to both sides".
Mr Hogan also said companies had to prepare for the reality that the UK is leaving the single market and customs union even if there is a deal.
He said the UK needs to "face up to the reality that no stable of new trade deals will offer as much opportunity or security as the one they're in now, the one that they wish to do with the European Union".