When we were teenagers, a dyslexic friend of mine, who was exempt from studying Irish in school, started using the slogan “tiocfaidh ár lá”. When I asked him what it meant, he said “Up the IRA.” To this day he’s still slagged off for it, though he wasn’t exactly wrong. The meaning of those four syllables, “our day will come,” is synonymous with dissident Republicanism, but also speaks to the innate hope among many for a united Ireland, not through violence, but through choice.

Last year, Mary Lou McDonald ended her first speech as the leader of Sinn Féin with an unscripted tiocfaidh ár lá. While allegations of IRA involvement plagued her predecessor, she promoted herself as part of a new generation: a Dublin woman with no involvement in the conflict. In that spirit, she is now calling for a border poll, claiming a referendum on a united Ireland is inevitable in the case of a no-deal Brexit.

British politicians have already admitted as much; Theresa May last month warned MPs that crashing out of the EU would risk a border poll and pose a real threat to the “precious union”. Even before Brexit, Jeremy Corbyn told the New Statesman in 2015 that he supported aspirations for a united Ireland. While Taoiseach Leo Varadkar accused Sinn Féin of being “disruptive” for pushing the issue of reunification, he was quick to remind people recently that his party, Fine Gael, is the “united Ireland Party.”

But what would unity really mean in Ireland today – and what is at stake?

Under the terms of the 1998 Good Friday Agreement, a border poll should be held if a majority would likely vote for reunification. Referendums would be held in both the Republic and Northern Ireland. The responsibility of calling the poll rests with the secretary of state for Northern Ireland, currently Karen Bradley, who admitted last year she didn’t know “people who are nationalists don’t vote for unionist parties and vice-versa.”

Brexit supporters have shown particular ignorance when it comes to the “Irish question”, with some suggesting that threatening Ireland with food shortages – a country that suffered famine under British rule – would be a good way to gain leverage. Jacob Rees Mogg advocated people be inspected on the border, as “during the Troubles.” Democratic Unionist Party leader Arlene Foster claimed a hard border never existed.

According to recent polls, 86 per cent of people surveyed in the Republic preferred a united Ireland to a hard border and 62 per cent of people in Northern Ireland believe that Brexit makes a united Ireland more likely. Reunification would mean Northern Ireland automatically remains in the EU.

The Good Friday Agreement recognised the constitutional will of the Irish nation to “unite all the people who share the territory of the island of Ireland, in all the diversity of their identities and traditions,” and that “a united Ireland shall be brought about only by peaceful means”, through consent "democratically expressed, in both jurisdictions in the island."

A united Ireland was always “the solution that dare not speak its name,” says Margaret Urwin, author of A State in Denial, a book about the British government’s collaboration with loyalist paramilitaries. But speaking about reunification used to lead to accusations of supporting the IRA. “It’s a breath of fresh air now people feel able to mention it,” she tells me.

Urwin reminds me that in 1918, before partition, Sinn Féin won a landslide victory by promising to leave their seats in Westminster empty. They would establish the revolutionary Irish parliament known as the Dáil instead, which sat for the first time one hundred years ago. The majority of Irish people effectively voted for independence. The North was isolated in its support for unionists. Now, despite a majority of people in Northern Ireland (56 per cent) voting to remain in the EU, their future is decided by Westminster.

Reunification would bring challenges. Northern Ireland receives a multi-billion pound annual subvention from Westminster. The EU is already discussing an emergency fund to protect Ireland’s growing economy from Brexit fallout. But some economic models predict reunification could actually lead to a boost in all-island GDP.

Parties new and old are now campaigning for seats both sides of the border. The Social Democratic and Labour Party in Northern Ireland voted last week to partner with the Republican party, Fianna Fáil. One Dubliner recently encouraged Catholics in Ireland to join the DUP, which opposes abortion and marriage equality, calling it the “only Christian party on the island of Ireland.” Cross-border activism to remove the abortion ban in Northern Ireland is also encouraging a more secular cross-island solidarity.

The murder of a loyalist man in Belfast last month, whose family blamed the Ulster Volunteer Force, is a reminder that the loyalist paramilitary group is still active. Many people forget that the biggest loss of life in a single day during the Troubles took place south of the border, when the UVF set off bombs in Dublin and Monaghan. Families formed a truth and justice campaign, Justice for the Forgotten, and are still pursuing answers in Belfast’s high court.

A 19-year-old who lives just south of the border, born after the Good Friday Agreement, told me that tensions in her area are already high, with bomb scares and talk of the New IRA. Her nearest cinema, hospital and shopping centre are across the border. A united Ireland is her dream, but she fears it will never happen.

Following a recent car bomb in Derry, a man whose teenage granddaughter had passed the vehicle shortly before it exploded spoke to BBC about his disbelief that young people are threatened with a return to violence. Peace walls still stand between communities. Unionists would need to feel secure and represented within a united Ireland. Divides would need to be healed, not hardened.

Some Brexiteers might not understand why the hope for reunification is fundamental to Irish politics. But they should understand that when it comes to a desire to remain within the EU, the island of Ireland is already united.