Irish opposition politicians have called for the publication of an internal report by the country's customs authorities discussing the potential impact of Brexit.
The document reportedly says that an open border between Northern Ireland and the Irish Republic will be "impossible" from a customs viewpoint.
The paper was seen by broadcaster RTÉ.
A final scenario of eight designated border crossing points for freight is described as "not inconceivable".
A Department of Finance spokesman said the report was an internal working paper prepared more than a year ago.
"Thinking has now moved on very significantly," he added.
"In May of this year, the Revenue chairman told a Joint Oireachtas Committee that Revenue is not considering any new customs posts."
'Suppressing important analysis'
Irish Prime Minister Leo Varadkar tweeted that the country was working to prevent a customs border.
A customs border would be expensive, a barrier to trade & an obstacle to peace. That's why we are working to prevent it, with support of EU.— Leo Varadkar (@campaignforleo) October 9, 2017
Fianna Fáil's Brexit spokesman Stephen Donnelly said the government had been "suppressing what I believe is the most important part of analysis by the state that we've seen".
"The government needs to publish this report immediately," he said.
In a speech last month the UK prime minister restated there would be no physical infrastructure at the border after Brexit.
Theresa May said that both the UK and EU had "stated explicitly" that they would not accept any physical infrastructure at the border.
That is in spite of the UK intending to leave both the customs union and the single market.