Boris Johnson has said he does not believe the Irish border would be affected if the UK left the European Union.
The mayor of London was in Northern Ireland on Monday just 48 hours after Prime Minister David Cameron's visit.
Mr Johnson announced a £62m order for County Antrim firm Wrightbus.
However, ahead of June's EU referendum he has discussed why he thinks the UK would be better off outside the European Union.
He told the BBC a Brexit would leave arrangements on the Irish border "absolutely unchanged".
"There's been a free travel area between the United Kingdom and the Republic of Ireland for, I think, getting on for 100 years," he said.
"There's no reason at all why that should cease to be the case."
Northern Ireland farming relies heavily on EU subsidies, but Mr Johnson said NI farmers would be no worse off outside the EU and "in many ways better off".
"You would be able to target the subsidy and we'd be getting money back from the EU that currently goes to Brussels and goes on heaven knows what," he said.
"We lose about £8.5bn to £9bn per year and we never see it again."
'Time for freedom'
He said he was taking a different position on the EU from Mr Cameron, because he did not think the prime minister had secured the reform that was needed.
"I think that there is every chance that the people will see that this is really their chance to get power back from Brussels," he said.
"Time for freedom folks is what I would say."
On Saturday the prime minister visited two businesses in County Antrim and urged Northern Ireland voters to stay in the EU.
"I would say it comes down to a very simple argument, which is do we want a greater United Kingdom inside the European Union with the safety, the strength and the prosperity?" he said.
"Or do we want a great leap in the dark?"