A customs frontier between Ireland and the UK is unavoidable in the event of a British withdrawal from the European Union, a conference heard yesterday.
Brussels-based solicitor Dr John Temple Lang of Cleary Gottlieb also said there would no longer be a common travel area between the two jurisdictions as there would be an "immigration frontier".
The conference, jointly organised by business body Ibec and the Irish Centre for European Law on the legal implications of a Brexit, was also told that Irish businesses are delaying investment decisions because of the uncertainty surrounding the vote on June 23.
"There is going to be a customs frontier between this country and the United Kingdom," Dr Temple Lang said.
"You can, I think, assume that there will be no customs duties or tariffs on the trade in goods, but there will still be a customs frontier. That is unavoidable."
He said it was "particularly unfortunate" that there would be a customs frontier for agricultural products.
"The reason for that is not because there will be duties or tariffs but because the United Kingdom will have to adopt a whole new agricultural policy about which we don't know anything," he added.
"Whatever that agricultural policy will be, it will involve different treatment from what now occurs when you export or import across the notional frontier between Ireland and the United Kingdom."
Earlier, Ibec director general Danny McCoy said the depth of the investment decisions that could unravel as a result of a Brexit vote is "unknowable".
He said businesses are already delaying decisions. He said there were knowable consequences for Ireland.
Blanaid Clarke, professor of corporate law at TCD, spoke of the impact on the financial services sector.
Prof Clarke pointed out that Irish banks have a sizeable and largely property-related exposure to the UK economy. She said the five retail banks have a total loan exposure of €64bn.
Former Attorney General Paul Gallagher said the UK was "sleepwalking" into the vote, and that a Brexit would cause immense damage to the EU.
Meanwhile, Japanese Prime Minister Shinzo Abe cautioned Britain that a vote to leave the European Union would make Britain less attractive for Japanese investors.
His intervention comes less than two weeks since US President Barack Obama bluntly warned Britain that it would be "in the back of the queue" for a trade deal with the United States if it dropped out of the European Union.
"Japan very clearly would prefer Britain to remain within the EU," Abe said during a visit to London. "Many Japanese companies set up their operations in the UK precisely because the UK is a gateway to the EU."