BELFAST -- As the United Kingdom prepares to leave the European Union on Friday, some are concerned about the impact on trade along the border between Northern Ireland and Ireland, and potential conflicts.

Northern Ireland will leave the European Union along with the rest of the United Kingdom on Friday evening. But the Republic of Ireland to the south will remain part of the EU.

The border between the two states was among the many complications faced during Brexit negotiations. Now, a hardening border between the regions could dredge up some complicated history. For 20 years, the hard-fought open border has maintained a kind of peace between the Republic of Ireland and Northern Ireland, regions with a violent history.

“We have what I call a cold peace,” said sociologist John Brewer.

Nationalist conflict in Ireland erupted into violence in the 1960s after years of an escalating tension between the Republic of Ireland and the U.K., of which Northern Ireland is a part. The violence would lead to the deaths of 3,600 people over three decades in what was known as “The Troubles.” Much of the violence occurred along the border, which had previously been open and fruitful.

Now, Brexit threatens a re-hardening of that border more than 20 years after the historic 1998 peace deal, which said people could hold both U.K. and Irish citizenship and the North could later vote to join Ireland if it wished.

The United Kingdom’s departure from the EU raises some uncertainty around trade between the Republic of Ireland and Northern Ireland, which will leave the EU, at least partially. The region will remain part of the Customs Union, following its rules and regulations on agriculture and manufactured goods.

“The challenge for many is -- is that not creating two borders, which would create a double burden, not just on business but on the society as a whole?” said civic activist Conor Houston.

But a seeming Brexit paradox has been raised: trading with the Republic of Ireland, an EU member to the south, could be easier than sending and receiving goods from fellow U.K. members England, Scotland and Wales.

But Niall McQuillan, the director of a steel business, is cautiously optimistic. He imports his product from around the world and sells it to the rest of the U.K. and Southern Ireland.

“We’re optimistic by nature,” he said. “This country, we’ve had adversity that everybody knows about, but we flourish. That’s what we do.”