Tánaiste Simon Coveney has admitted checks on goods moving between the UK and Ireland will be necessary in the event of a no-deal Brexit.
In a private conversation caught on tape, Mr Coveney indicated ministers should not talk about the checks publicly for fear of a backlash.
The admission came as British Prime Minister Theresa May suffered a historic defeat in the House of Commons on her EU exit deal.
"It is clear the House does not support this deal, but tonight's vote tells us nothing about what it does support," the prime minister said.
She now faces a motion of no confidence in her government.
After Mrs May lost by 432 to 202, EU President Jean-Claude Juncker bluntly said the risk of a hard Brexit had increased and "time is almost up".
However, during a private conversation with Transport Minister Shane Ross, the Tánaiste confirmed a form of checks will be introduced in a no-deal "but we can't get into where they'll be at this stage".
"But once you start talking about checks anywhere near the Border, people will start delving into that and all of a sudden we'll be the Government that reintroduced a physical border on the island of Ireland," he said. Mr Coveney added that the border checks "could be at sea".
The admission came after Mr Ross bungled a question from the Irish Independent on whether in a no-deal scenario a truck could travel from Scotland to Northern Ireland and then on to the Republic without any checks.
Mr Coveney told this newspaper: "We are not planning to put checks on the Border."
Tánaiste Simon Coveney and Minister Shane Ross were talking privately at the end of press briefing without realising the microphone was still on:
Shane Ross: “Yeah. The border one, should I not have said that?” (This is a reference to a question Mr Ross was asked about whether a truck travelling from Scotland by boat to Larne could then proceed to the Republic without any checks. During the press briefing, he replied: “I would anticipate that there would be checks.”)
Simon Coveney: “Yes, but we can’t get into where they’ll be at this stage. They could be in the sea, they could be... But once you start talking about checks anywhere near the border, people will start delving into that and all of a sudden we’ll be the government that reintroduced a physical border on the island of Ireland.”
Shane Ross: “Yeah, but I didn’t know what to say.”
Meanwhile, Mr Coveney says the EU is no "no mood" to change the Withdrawal Agreement significantly after Mrs May's defeat in Westminster on Tuesday.
He added that he believes the UK Prime Minister will survive the vote of no confidence tabled by opposition leader Jeremy Corbyn.
On RTE's 'Morning Ireland' programme on Wednesday, Mr Coveney said: "There is so much still to happen in Westminster in the next few days before it becomes clear what Britain is asking for.
"It's hard for us as EU leaders to respond in a helpful way if we still don't know what they are asking for.
"We need to hold our nerve this week. There's a motion of no confidence today, I believe that will be lost, and my understanding is that the Prime Minister's intention then is to try and reach across parties to try and find a way of getting a majority in Westminster to support a new approach.
"When we understand what that new approach is, which we get next Monday, we'll try and respond in a helpful way."
Mr Coveney noted that the Withdrawal Agreement was signed by 28 governments including Britain.
"I don't think the EU is in any mood to change the Withdrawal Agreement significantly at all," he added.
"We don't even know what the ask is, we'll have to wait and see what they're asking for.
"Those who have rejected the backstop do not have alternative solutions in terms of how we provide reassurance (on the border)."
He added: "If there isn't a future relationship that resolves the border issue, then the backstop will kick in which involves minimal checks on goods moving from Great Britain into Northern Ireland.
"The position is we support the backstop. The Irish Government have made very clear that we are not going to start talking about alternatives to the backstop. It took us two years to negotiate an approach that can provide the guarantees that we have given to people.
"We're not going to allow physical border infrastructure to reappear."
Mr Coveney concluded saying Ireland will not block any UK request to extend Article 50 but there needs to be a plan.