Two activists with the EU flag and Union Jack painted on their faces kiss other in front of Brandenburg Gate in Berlin to protest against the British exit from the European Union, June 19, 2016. Reuters/Hannibal Hanschke

As Irish soccer fans crammed into a central London pub last week to watch their country's opening European Championship match, they were greeted at the door by fellow expatriates urging them to help keep Britain in the European Union.

The canvassers, among the 20,000 Irish workers who move to Britain each year to join an estimated half a million already living and working there, are part of an unprecedented campaign by Ireland to avoid the collateral damage of a Brexit.

From Prime Minister Enda Kenny to enthusiastic volunteers like Bronagh McCloskey, Irish politicians and workers have been hitting the stump in the towns and cities of their nearest neighbor in a manner no other country would dare contemplate.

Irish citizens living in the United Kingdom have the right to vote in U.K. elections and referendums, even though Dublin won its independence from London almost a century ago. The right long predates the entry of both countries in 1973 into the European Economic Community, which has evolved into the EU.

"I'm always loathe to be that person to say if 'x' happens I'm going to leave the country but this is the first time I've ever thought it," said McCloskey on why she has handed out leaflets at pubs, train stations and Gaelic football pitches on behalf of the pro-EU, British-based Irish4Europe civic group.

McCloskey, 27, a public affairs worker, shares the fears of the Irish government that a British vote to leave the EU in Thursday's referendum would damage trade, hurt the economy on both sides of the Irish border and have a devastating long-term impact on her native Northern Ireland.

Dublin officials are particularly worried about the consequences for British-ruled Northern Ireland, which has the only land frontier between the United Kingdom and the rest of the EU and was marked by military checkpoints until a 1998 peace deal ended three decades of sectarian violence.

British 'Leave' campaigners say Brexit would not endanger the Northern Irish peace process or damage bilateral trade.

Kenny, who last week danced with older Irish expat voters in a Liverpool bingo hall as he made the case for the 'remain' camp, has put at the center of his message concerns that any new border restrictions could endanger peace in Northern Ireland.

Peace Agreement

"This momentous decision is as important for the future of this island as when we all voted for the Good Friday Agreement," he told university students in Londonderry last week, referring to the peace accord that ended bloodshed between Catholic Irish nationalists seeking a united Ireland and their Protestant rivals who want to keep Northern Ireland British.

While U.S. President Barack Obama came to Britain to speak out strongly against Brexit, Ireland is the only fellow EU country that has been actively encouraged by pro-'remain' British officials to have a public opinion on the issue, two diplomatic sources said.

Given the special status of Irish citizens in the U.K., Dublin feels it has a legitimate voice and most of Kenny's cabinet, as well as the main opposition party leaders, have traveled to Britain in recent weeks to make sure it is heard.

Citizens of Cyprus and Malta, which are in the Commonwealth, are the only other EU nationals able to vote in the referendum.

McCloskey says she will be encouraging friends on social media to make sure they get out and vote while Irish4Europe and other community groups are encouraging big Irish employers to give their staff half days so they can get to polling stations.

"There is a strong 'remain' sense among the Irish community in the UK and in London in particular ," said Irish4Europe co-chair Grainne Melon, a barrister who has worked in London for the past seven years.

"We feel that we are making a difference in getting Irish people aware of the issues for Ireland and also in reaching out to people in the UK generally who have an affinity for Ireland."