Passport controls will not likely be required on the border between Northern Ireland and the Irish Republic after Brexit, the country's prime minister Leo Varadkar has said.
The former Irish president Mary McAleese had asked how there would be a differentiation between Irish citizens and other EU nationals without some kind of ID checking.
If the Common Travel Area (CTA) stays in place after Brexit, British and Irish citizens will be able to travel freely across between the north and south of the island.
But McAleese told RTE: "My view is that sooner or later pressure will come on to make it an ID card phenomenon."
But Varadkar told the Irish broadcaster that the post-Brexit restrictions that the UK wanted to impose would only apply to limiting workers' rights and benefits rather than physical checks.
"In life and politics nothing is 100% certain, but I am very confident that there won't be passport controls between Northern Ireland and Ireland. We want to stay in the Common Travel Area, which allows people to travel freely between north and south and Britain and Ireland.
"The British government wants that too and so do our European partners.
"So while there will be a big debate and difficult negotiations around issues such as trade, around issues such as the financial settlement, the fact that Dublin, Belfast, London and Brussels want to continue passport-free travel between Northern Ireland and Ireland gives me absolute assurance that that won't be the case.
"I understand former president McAleese's concerns but it is one area that I am very sure about and that's that there won't be a requirement to produce a passport to travel to Northern Ireland."
The British government has already argued that it would oppose any new physical monitoring either through CCTV cameras or number plate recognition systems, although the EU chief negotiator has viewed the idea of invisible borders with scepticism.
Varadkat told RTE: "The reason why I can be confident that there won't be passport controls between Northern Ireland and Ireland is the fact that nobody is looking for them."