Irish prime minister Leo Varadkar has said he wants free movement of people, goods and services between the UK and Ireland after Brexit.
Speaking on his first official visit to Northern Ireland, Mr Varadkar said he did not want "an economic border", repeating his belief the onus was on advocates of a hard Brexit to come up with detailed proposals on a new agreement.
"They've already had 14 months to do so," he said.
Mr Varadkar said he did not believe hardline Brexiteers could come up with detailed proposals to replace current trade agreements and said it was time to start talking "meaningfully" about a solution that would "work for all of us."
In a speech at Queen's University, Belfast, Mr Varadkar said: "I do not want an economic border on the island. Nor do I want one between Ireland and Britain. And by an economic border I'm not talking about currency or variations in tax rates, I am talking about a barrier to free trade and commerce.
"However, there are people who do want a trade border between the United Kingdom and the European Union and, therefore, between Ireland and Britain and, therefore, across Ireland.
"These are of course the advocates of a so-called hard Brexit. I believe the onus is on them to come up with proposals for such a border and convince us and convince you... such a border would be in your interests and that these borders would not be barriers to trade and commerce.
"They have already had 14 months to so, which should have been ample time to come with detailed proposals. But if they can not , and I believe they can not, then we can start to talk meaningfully about solutions that might work for all of us."
The prime minister said that as recently as last November, his government had sat down with a north-south ministerial council, which included the DUP and Sinn Féin, and had agreed on four principles on how they could work together after the UK leaves the EU. This included protecting the free movement of people, goods and capital and services, he said.
Mr Varadkar is on a two-day visit to Northern Ireland to discuss north-south relations and will later be meeting with the DUP leader Arlene Foster.
The face-to-face comes days after a row over the Irish-UK border between the DUP and Mr Varadkar after he insisted there was no proposal that the Irish Sea should become the new frontier with the UK after withdrawal from the European Union.