The EU's future border with the UK after Brexit is an almost 500 kilometre line straddling Northern Ireland in the UK, and the Republic of Ireland in the EU.
Right now, it's invisible. People and goods can cross back and forth freely.
But with the UK planning to leave the single market and customs union, it could all change. And many locals fear what may happen.
"They don't want to go back to this situation where the roads are going to be closed. And if there's a hard border I think it will cripple the country," says border trader Eamon Fitzpatrick.
An estimated 30,000 people cross the border each day and free movement is key to companies operating on the island. Guinness, for example, ships beer from Dublin to be bottled in Belfast and back again.
The UK does not want to see a hard border and it has suggested using electronic tagging of goods and waivers for small businesses.
"It would suit everyone if Britain remained in the customs union or the single market. If they're not, we're going to have to work out what will that new customs relationship be. But it cannot be a customs relationship which goes back to the 1970s or to the 1980s," Irish MEP Brian Hayes, of the European People's Party told Euronews.
He says Dublin would not accept any physical infrastructure reinstated at the border.
Bringing down checkpoints was a key aspect of the 1998 peace deal between Catholic nationalists seeking a united Ireland and Protestant unionists wanting to remain part of the UK. The agreement ended decades of conflict that killed more than 3,600 people.
Nobody wants to see a return to physical border checks. A Sinn Fein lawmaker has warned these could anger people and lead to "civil disobedience".
Back in Brussels, MEPs fear Brexit could revive sectarian tensions and undermine peace on the island.
"I am concerned about the language and the use by some political parties, especially Sinn Fein, of the ramping up of their particular brand of nationalism for the purposes of their own interest," said Brian Hayes, the Irish MEP
"So we've got to be very careful about this. We cannot allow community tensions in Northern Ireland to increase as a consequence of this. And it's the responsibility of all of us in the EU and in the UK to get to a good settlement of these issues."
Any new type of border on the island would be entirely untested.
Additional reporting by Nial O'Reilly