Taoiseach Enda Kenny warns custom posts could return to Irish border if UK quits EU
Custom controls could have to be set up at the Irish Republic's border with Northern Ireland in the event of a Brexit vote, the Taoiseach has warned.
Enda Kenny urged Irish people living in Britain to vote Remain next month, saying the Republic would be one of the worst affected countries should the UK leave the European Union.
Speaking in London, Mr Kenny said Irish people make up a "substantial" part of the electorate and urged them to vote.
Irish citizens living in the UK will have a vote on June 23, alongside British citizens who are living in Ireland.
Campaign Group Irish 4 Europe estimates around half a million first generation Irish live in England, Scotland and Wales, and say the number rises to the millions when second and third generation are considered.
Mr Kenny spoke as he prepared to watch his home county of Mayo take on London in a Gaelic Football Championship match in Ruislip.
A firm supporter of the Remain campaign, Mr Kenny said: "We would say, with particular reference to the Irish people living in Britain, this is a really important decision.
"Its outcome would affect people in Northern Ireland, it would affect Ireland itself and obviously will have an impact upon the European Union for many years to come."
Asked what might happen in the event of a vote for Britain to leave the EU, he said there would have to be "significant, complex and difficult negotiations".
He added: "Ireland in Europe would still stand by Britain being a member of the Union and of its importance, but I have no idea what other European countries, how they would look at Britain whether they decide to leave, given the fact that we've come a long way since the 1970s.
"So, whether there would be border controls or custom controls, these things are a possibility but obviously they would require some very serious negotiations and my preference for the Irish electorate who have a significant part in this referendum is to vote to stay, for Britain to stay as a strong and central member of the European Union for the future." Mr Kenny said Ireland and Northern Ireland would be "most adversely affected" in the event of a Brexit vote.
Crowds of supporters, decked out in the green and red of Mayo, were handed leaflets urging them to vote remain as they entered the GAA ground.
The idea of border controls would be "awful", match-goer Barbara McKenna said.
The 26-year-old, who works as a nurse in London but is originally from Monaghan, said she intends to vote Remain.
Ms McKenna said: "I think Britain is better off in the EU and it allows countries to work together.
"At home in Ireland I am beside the border, so if I have to go through controls every time I wanted to cross, it would be like moving back to the 1970s."
Michael English (47) said Britain is strong enough to prosper regardless of whether it stays in or leaves the EU.
Mr English, who is from Mayo but lives in London, said he does not intend to vote.
"Britain is a big, strong country. If it stays in it'll be fine, if it leaves it'll be fine. Either way, it doesn't matter."