Overlooking the Lunch Lounge cafe on Castlereagh Road in Loyalist East Belfast, three gunmen pose with automatic weapons and offer their unwavering support ‘For God and Ulster’. The mural is a reminder of darker days; the Troubles, when Northern Ireland was riven with sectarian hatred.

Amid the talk of no deal and hard borders, does Brexit really have the power to hand Northern Ireland’s future back to the gunmen?

History shows what would likely happen to any fixed infrastructure on the border. “If something looks like a target it will be treated as such,” says a spokesman for the Police Service for Northern Ireland (PSNI). A former Special Branch Officer agrees: “Any infrastructure on the border will be a target, without a shadow of a doubt.”

Rancour and division remain in Northern Ireland. The imperfect peace has left “a disappointing variety of normal” compared to the rest of the UK, in the words of one security expert. Certainly levels of violence are down: in 2018 there were 39 shootings and 17 bombings, both down slightly from the previous year, resulting in 50 casualties and two deaths. But while the allure of paramilitary groups has dimmed, as their capability and calling has reduced, they haven’t gone away you know, and the polarisation of politics means they have a greater audience for their messages. Young hot-heads will always listen to war stories.

Such talk is quickly dismissed by locals in the Lunch Lounge cafe.

“Brexit won’t make any difference to us,” says an octogenarian diner buttering a fruit scone. But not everyone is so sanguine. A lot will depend on whether Unionists perceive a threat to the union, says a security source. If they feel disenfranchised, Loyalist paramilitaries could be encouraged to take to the streets.

Those keen on maintaining, and defending, the link between Northern Ireland and Great Britain work in areas of national infrastructure – such as power generation and transportation – to a much greater degree than those with Nationalist sympathies. Concerted industrial action, albeit short of actual civil disobedience, could still cause headaches in Westminster.

Doug Beattie, a member of the Stormont Assembly for the Ulster Unionist Party, describes himself as an optimistic pessimist. Speaking in Portadown, a town once under the spell of the Loyalist Volunteer Force, he expresses support for the teaching of the Irish language, same-sex marriage and abortion. But his sunny disposition soon darkens.

Pointing out that 56 per cent of people in Northern Ireland voted for Remain in 2016, he fears both the DUP and Sinn Féin see advantages in whipping up tensions. “They’ve both done it before,” he says. The DUP play on the sovereignty issue, he believes, and Sinn Féin, a party historically cool towards the EU, senses another opportunity to push for a united Ireland, so gripes about leaving. They may even orchestrate civil disobedience in border towns such as Newry, Crossmaglen and Londonderry so as to agitate for a referendum, he warns.

“If we exit on a no deal basis then [Sinn Féin’s] call for a border poll will really take off. And it’s difficult to say they can’t have it. And if we do it for Northern Ireland, there’s a chance we may have to do it for Scotland. A no deal Brexit could see the breakup of the union.”

The Falls Road in Republican West Belfast holds a totemic position in the history of Britain’s involvement in the Troubles. Locals are wary of outsiders, and the Sinn Féin offices, still sporting the mural to Republican hero Bobby Sands – proclaiming ‘Our revenge will be the laughter of our children’ – are only accessed after much explanation and the unlocking of doors.

The party says there are only two ways to avoid a hard Brexit on the island of Ireland. First, the north (it does not use the term ‘Northern Ireland’) should be given special status and stay within the EU structures. The second way would be to unite the island after a referendum. Neither are remotely palatable for Unionists.

Sinn Féin’s leader, Mary Lou McDonald, said in a statement to the Telegraph: “Ireland’s people, Ireland’s economy and Ireland’s peace process all need to be protected as we go forward.

“We need to say clearly to the British that if they wish to Brexit then that’s a matter for themselves but any Brexit agreement needs to recognise, understand and protect the people, the economy and the peace process on this island.”

The British government is keen to dampen any speculation of increased security preparation. The PSNI currently grades the threat from Dissident Republicans as ‘severe’ and says the government gave permission for an extra 300 officers to be recruited as a one-off because of the uncertainty around Brexit. Beyond that all the spokesman would say was “it’s a political decision. Everyone’s watching with bated breath”. The army and Northern Ireland Office would not discuss the issue.

However, security insiders suggest that rather than fixed infrastructure or a greater presence by the police or military, the security response to a no deal Brexit is expected to be more subtle. Intelligence-led work by the police, MI5, National Crime Agency and others is likely to increase in intensity, if not visibility. The check points were there to tackle terrorism, says Mr Beattie, “and even then they didn’t work”.

Like the rest of the UK, whichever way Brexit goes there will be people left feeling hard done by in Northern Ireland. The difference is that with the Stormont Assembly suspended for the last two years there are few political mechanisms through which the inevitable issues can be resolved. Border infrastructure in the event of a no deal Brexit is most unlikely, but civil disobedience, with the attendant risk of spiralling into greater violence, is not. Agendas abound, hidden and otherwise, and paramilitaries still lurk. As one security source says: “The devil continues his work in the shadows”.