Taoiseach Leo Varadkar has said it will be difficult to avoid a hard border if no deal is in place when Brexit takes effect.
It is the first time he has effectively acknowledged border infrastructure could return.
Mr Varadkar said there was "not much room for renegotiation" of the proposed Brexit deal, adding that it would be "very difficult" to avoid a hard border if no withdrawal deal was in place when the UK leaves the European Union next March.
"If you start trying to amend it or unpick it you might find that the whole thing unravels," Mr Varadkar said of the withdrawal agreement struck this week. However, British prime minister Theresa May faces a struggle to get the deal through the House of Commons.
Mr Varadkar was speaking at a housing launch in Dublin on Friday.
"I think in a no-deal scenario it would be very difficult to avoid a hard border," he said. "As Ireland [is] remaining part of the European Union, we would no doubt be required to implement European law and the United Kingdom, having left the European Union, would seek to join the World Trade Organisation and they would have to implement World Trade Organisation rules.
“So those hard Brexiteers who say that somehow just through good political will you can avoid a hard border, that doesn’t make sense. Are they really saying that they would join the WTO and the first thing they would do would be to refuse to implement the WTO rules?
“Are they really saying to us that we would stay in the European Union and not protect the single market? The only way we can avoid a hard border is by an agreement, an agreement that covers customs and regulations. We have that now.”
While Mr Varadkar said Ireland was continuing to make preparations for a no-deal Brexit, he said more and more people in the UK could support the current deal as such a prospect approached.
“I know, looking at things today, it looks like it is going to be very difficult to win the vote in the House of Commons but I think as reality kicks in, as the precipice approaches, you might find more and more people willing to support this deal as the best possible outcome that can be achieved.”
Mrs May has “shown enormous resilience in the past”, he said.
Mr Varadkar also said he had yet to hear any alternative proposal to the withdrawal agreement that avoided a hard border, protected citizens’ rights and allowed “trade to continue normally, across Ireland, Britain and Europe”.