The murals outside the small offices of dissident republican group Saoradh give the first hint not all is well in Londonderry, Northern Ireland's second-biggest city, also known as Derry.

One sign — featuring militants in face masks holding assault rifles — commands passersby to "join the IRA," the initials synonymous with the Irish Republican Army, which was once known as the most notorious paramilitary group in the region, deploying guns and bombs to fight for the reunification of Ireland.

"RPG!" a boy shouted recently as he walked along in an elementary school group, referring to the rocket-propelled grenade launcher featured prominently on another mural.

Such is life in a region shaped by decades of the conflict known as the Troubles. A longstanding guerrilla war of the late 20th century, it pitted paramilitaries loyal to Britain and Ireland against each other, and hardline Irish nationalist groups against the British Army. Allegiances were often — but not always — divided along Catholic and Protestant lines.

Today, Northern Ireland is grappling with a future outside of the European Union, wondering how Brexit will affect tensions in border communities like this one.

"If you put up customs posts along the Irish border, people will shoot at them. There is no doubt about that," said Eamonn McCann, a Londonderry-based author who lived through the Troubles and who has written about the conflict extensively.

Eamonn McCann, a well-known Londonderry-based author and commentator, Is blunt about the perils if Brexit brings a hard border. (Stephanie Jenzer/CBC)

Many people in Northern Ireland still consider the peace process an ongoing course of action.

"I'm pretty sure people within Canada think the so-called war has ended after the Good Friday Agreement was signed, [but] that's not the case," Saoradh spokesman Patrick Gallagher told CBC.

The 1998 deal sought to give Irish nationalists and British unionists equal footing in Northern Irish politics and bring lasting peace to the region.

"The fact is Ireland's still occupied," Gallagher said.

He says as long as the "British occupation" continues, Irish republicans here will be willing to take up arms against Britain.

Saoradh spokesman Patrick Gallagher says as long as the 'British occupation' continues, Irish republicans here will be willing to take up arms against Britain. (Stephanie Jenzer/CBC)

Current-day paramilitary groups are also seeking to capitalize on the divisions laid bare by Brexit, according to a former senior police officer.

"They have openly said they see this as a recruiting platform," the former assistant chief constable in Northern Ireland, Alan McQuillan, told the BBC. "They believe they can build their campaign from this.

"There's a real threat," he said.

'Almighty' car bomb

The ever-present threat of violence came into stark focus here on Jan. 19, when a hijacked pizza delivery van was parked outside the courthouse in Londonderry and blown up.

Jenni Doherty, who manages a bookstore around the corner, said she heard an "almighty" bang.

"I didn't know it was a bomb but thought: 'What in the name of God is that?' " she said.

An IRA slogan is spray painted on Londonderry’s 17th-century walls that make up one of the city’s most distinguishable and historical features. (Stephanie Jenzer/CBC)

Police had been tipped off and were able to evacuate some nearby buildings. No one was hurt.

But the explosion was powerful enough to shatter nearby windows and leave the vehicle's outline melted into the pavement.

Police release CCTV footage of explosion outside courthouse on Bishops Street in Derry/Londonderry last night. <a href="https://t.co/tqzqBdCZnv">pic.twitter.com/tqzqBdCZnv</a> —@PSNIDCSDistrict

It was the first car bombing this city has suffered in years and served as a jarring reminder of the type of attack Londonderry hadn't regularly seen since the Troubles.

"It was scary," said Conor McGinty, a local singer who drove by the hijacked van with his fiancée and one-year-old son only minutes before it blew up.

McGinty, 24, acknowledges he's too young to recall the worst of the Troubles, such as Londonderry's Bloody Sunday massacre in 1972 when British soldiers opened fire on peaceful protesters. Fourteen people died.

Conor McGinty stands in front of the Londonderry courthouse where the bomb exploded on Jan. 19. (Stephanie Jenzer/CBC)

"We went through a period of nearly 20 years of peace, and it's been torn apart really by Brexit," McGinty said, fearing the car bomb would be a sign of things to come.

What Brexit means here

A majority of voters in Northern Ireland — 56 per cent — opted to remain in the EU in the 2016 Brexit referendum. However, as all of the U.K. is set to leave on March 29, the region finds itself in a unique predicament.

After Brexit, it will share the U.K.'s only land border with the EU, the current flashpoint in a difficult divorce.

The invisible 499-kilometre frontier that separates Northern Ireland from the Republic of Ireland, an EU member, remains the thorniest issue preventing parliamentarians in London from ratifying the withdrawal agreement.

European leaders and U.K. Prime Minister Theresa May had agreed to the Northern Ireland backstop — a series of measures to prevent the reimposition of customs checks — but British MPs resoundingly rejected it in January, fearing it would keep the U.K. tied the EU regulations for years after Brexit.

British Prime Minister Theresa May speaks with business representatives in Belfast on Tuesday. (Liam McBurney/Associated Press)

Now, May herself is seeking changes to the backstop and trying to reassure residents of Northern Ireland about their future.

Her commitment to delivering Brexit without a hard border "is unshakeable," she told a Belfast business crowd on Tuesday.

Since becoming prime minister in 2016, May has only rarely travelled to Northern Ireland. This week's visit signalled an attempt to quell deepening fears that Brexit cannot be achieved without new customs infrastructure at the border.

Addressing the border issue and maintaining the peace remains the EU's "top priority," European Council President Donald Tusk said Wednesday, standing beside Leo Varadkar, the Irish prime minister.

Tusk warned there's a "special place in hell" for those who promoted Brexit without a plan to leave the EU "safely."

'The law of unintended consequences'

While no one wishes the return of a visible frontier, a solution to avoid one would need to be approved by leaders in London, Dublin and Brussels.

A separate political crisis in Belfast — which has left Northern Ireland without a functioning regional government since January 2017 — only exacerbates a feeling of helplessness among residents as Brexit day approaches.

"The law of unintended consequences has been one of the main laws of Irish history going back centuries," said McCann said in an interview.

"Nothing works out the way people expect it to work out."

Kathleen Gillespie’s husband was 42 years old when he was killed in an IRA bombing in Londonderry in 1990. Watching footage of January’s bomb has her fearing for others who narrowly escaped 'the pain I felt.' ( (Stephanie Jenzer/CBC)

Few people know about violence at the border more intimately than Kathleen Gillespie.

"I would not wish anybody to suffer the pain that I suffered. And to see it starting up again doesn't bear thinking about," she said at her Londonderry home.

While the British Army operated border checks during the Troubles, Gillespie's husband Patsy worked as a cook at a military facility a few minutes' drive from their house. His role led republican extremists to consider him a "legitimate target of war," his widow said.

On Oct. 24, 1990, IRA members held Kathleen Gillespie and her children hostage while Patsy was chained inside a vehicle loaded with 450 kilograms of explosives. He was forced to drive toward the border and let the bomb go off.

The massive blast killed him and five nearby soldiers. Patsy Gillespie's remains could only be identified using pieces of the grey cardigan he had been wearing and the flesh attached to it.

"One of the hardest things I've ever had to do in my life is to tell my son on his 18th birthday on the phone that the IRA had murdered his daddy," Kathleen Gillespie said as she shed a tear, more than 28 years later.

Present-day IRA

A group calling itself the IRA reared its head again recently, claiming responsibility for the Jan. 19 car bomb attack.

The hardline republican group vowed to continue to "strike at Crown forces personnel and their imperial establishment," using civil war language to describe U.K. symbols in Northern Ireland.

"All this talk of Brexit, hard borders, soft borders has no bearing on our actions and the IRA won't be going anywhere," read its statement published in the Belfast News Letter.

"Our fight goes on."

Londonderry is home to some of Northern Ireland’s most iconic murals, including the famed You Are Now Entering Free Derry sign, first painted in 1969. (Stephanie Jenzer/CBC)

But the frontier's newfound attention can't be so easily overlooked.

The leader of the dissident republican group Saoradh recently said Brexit would bring the border — and resistance to British "occupation" — into sharp focus.

"Brexit is showing up the fact that the island is divided, that we do have a border," Saoradh chairman Brian Kenna told the Irish Independent. He was convicted of IRA membership in 2017.

Pro-Britain politicians who campaigned for Brexit in Northern Ireland deny the EU breakup has any direct link to recent violence.

In fact, continued debate about the backstop shows how much this region means at the Westminster Parliament, according a Londonderry city councillor.

Graham Warke, a Democratic Unionist councillor, served in the military and guarded the border during the Troubles. (Stephanie Jenzer/CBC)

"We went into Europe as part of the U.K. all as one and we'll leave all as one," said Graham Warke, a Democratic Unionist councillor whose party's 10 MPs provide May's Conservatives with a majority government.

Warke also served in the military and guarded the border during the Troubles.

"We were stopping the bombs coming in over the border. We were stopping civilians getting murdered by these terrorists," he said.

"I wouldn't want to be seeing that again."