London (CNN) On Tuesday, the UK's lawmakers return to Westminster with the fog of tragedy hanging in the air.

Early Tuesday morning, Irish media reported that a dissident Republican group, the New IRA, had accepted responsibility for the murder of Lyra McKee

Her death, which took place following an evening of unrest in Derry on Thursday night , should be a reminder that the risk of violence on the island of Ireland is very real and that the political situation surrounding it is not a game.

The words that have been used to describe Thursday's violence underscore the tension in Northern Ireland. The police -- the primary target of the New IRA's gunfire on Thursday -- have said that McKee's killing was a "terrorist attack." The New IRA have apologized, saying that McKee was "tragically killed while standing beside enemy forces."

In London, Northern Ireland can feel remote. The words "Britain" and the "UK" are often used interchangeably, explicitly ignoring the fact that the United Kingdom of Great Britain and Northern Ireland is a union of four countries.

Also remote is the reality of Northern Ireland's recent history and the fact that the prospect of sectarian violence is a real concern for some living on these islands.

Terrorism is today largely defined by extremist groups, be they white supremacists or radicalized Islamists. To many people living in England, Scotland and Wales, the idea of acts of terror carried out by Northern Irish republicans is alien. After all, it's been 21 years since the Good Friday Agreement was signed in 1998.

But McKee's murder should be a wake-up call for everyone in the UK that in Northern Ireland, instability can quickly turn to protest which can quickly to violence which can quickly turn to death.

And there is plenty of instability in Northern Ireland right now. The national assembly at Stormont has not sat for over two years, following the collapse of the power-sharing agreement between the governing parties, republican Sinn Fein and the Democratic Unionist Party.

This leaves the province without a devolved government and having to hear talk of London imposing "direct rule" over it. Direct rule is something everyone wants to avoid, as it would almost certainly lead to more violence.

And the future of the island as a whole is uncertain because of the unique problems being caused by Brexit.

Lyra McKee pictured at TEDxStormont Women in 2017.

The single most contentious issue in the whole Brexit process has been solving what happens to the border between the Republic of Ireland and Northern Ireland.

The Republic is an EU member state and has no intention of that changing any time soon. Northern Ireland will leave the EU along with the rest of the UK, however that eventually happens.

Without an agreed deal on customs arrangements, there would be a need for customs checks on the border between the two nations. This is why British Prime Minister Theresa May's deal with the EU had something called the "backstop" in it -- an emergency measure to ensure that the border remains frictionless.

The idea of any infrastructure being put up on the border terrifies politicians in Dublin and Belfast.

The logic goes that if a camera or check points appear, they will quickly get torn down.

Guards would need to be put in post to protect the infrastructure, who then face the prospect of violence. This situation could quickly escalate and lead to police or even soldiers on the border.

But ever since the backstop was introduced last year, it has been subject to bitter political arguments. English Brexiteers hate it as they think it would stop the UK from having an independent trade policy, and Northern Ireland's DUP rejects it as it thinks it puts the integrity of the United Kingdom at risk.

And while remainers and soft Brexiteers don't specifically hate the backstop, their solutions to Brexit are soft enough to not require the backstop, so they have voted against May's deal, too. Though their commitment to no border on the island of Ireland is obvious, the lack of clarity that their repeated voting down of May's deal while not agreeing on a credible alternative does nothing for Northern Irish stability.

It is far too often forgotten in Westminster that in 2019, a dissident republican threat still exists within the borders of the UK, and they are only too happy to exploit instability or political chaos to inflict violence on police officers and innocent members of the public in order to further their cause.

The New IRA are criminal thugs who are spoiling for a fight with or without Brexit.

But when MPs return to Westminster on Tuesday to address both this tragedy and other parliamentary business, it should be hammered home to them that the instability caused by political vacuums can have real-world consequences.