The centenary of Northern Ireland is prompting reflections on the past, evaluations of the present and predictions about the future.
Demographic change, recent elections and Brexit have all fuelled an increased debate about the potential for constitutional change.
In a series of reports for PM on BBC Radio 4, I’ve been looking at the state of the union in Northern Ireland.
What does the Good Friday Agreement say?
The deal says Northern Ireland will remain in the union as long as most of its people wanted it to be.
But it includes a potential future pathway to a united Ireland through a referendum, also called a border poll.
The text which in UK law says the Northern Ireland secretary “shall exercise the power [to call a referendum] if at any time it appears likely… that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom”.
How is it being analysed?
The researchers stress they do not believe a border poll is imminent.
But Dr Alan Renwick from University College London, who chairs the group, says now is the right time to look at the issues because “careful, considered examination” will become more difficult if the possibility of a referendum rises.
The agreement does not spell out how the Northern Ireland secretary would decide if a majority for a united Ireland “appears likely”.
The working group has identified several sources of evidence.
It says the most significant are election results; opinion polls and surveys; and any votes in the Northern Ireland Assembly on whether to have a border poll.
Other factors include demographic data, such as the number of Protestants and Catholics recorded in the 2021 census; and “qualitative forms of evidence”, including focus groups and conversations with community leaders.
The academics believe the most significant of these are election results (particularly votes cast), opinion polls and any assembly vote.
The report sets out three different approaches to referendums, which the group thinks could be considered.
One would involve a referendum being held after a plan for a united Ireland is worked out as fully as possible.
The other two models would see a border poll taking place before such detailed proposals were formulated.
But the academics say there would need to be a plan for a process after the vote, to design the form of a united Ireland.
What do elections and opinion polls show?
In the last four elections – between 2017 and 2019 – unionist parties have averaged about 43% of the vote.
In 1921, about 66% of voters backed unionists.
In the elections since 2017, nationalist parties’ share has ranged from 36% to 40%.
Parties which are neutral on the constitutional question are now backed by about 20% of voters – twice the share they had 10 years ago.
There has been a range of opinion polls and surveys in recent years, with different sides in the debate often emphasising various findings.
How big a factor is Brexit?
Fifty-six percent of people in Northern Ireland voted to remain in the EU.
Gerry Carlile, from the civic nationalist organisation Ireland’s Future, says the group grew “organically” after the Brexit vote.
“People were having conversations with each other, over coffee, at the side of football fields, dropping off kids at schools,” he says.
“We had deep concerns about Brexit – particularly as Irish citizens who live here and felt our Irish citizenship was always protected because of EU membership.”
Brussels has agreed that Northern Ireland would automatically re-enter the EU if there was Irish unity in future.
Mr Carlile, who is an entrepreneur, also believes Brexit could be “catastrophic” in economic terms.
However, Dr Esmond Birnie, who is a senior economist at Ulster University and a former assembly member for the Ulster Unionist Party, believes the negative effects of Brexit may be exaggerated.
He says: “I think Brexit represents disruption but the scale may be much less than was forecast in 2016.”
Unionist parties are strongly opposed to the Northern Ireland Protocol – the post-Brexit trading arrangement which in effect has created a trade border with the rest of the UK.
What is the thinking in Republic of Ireland?
The Taoiseach (Irish prime minister) Micheál Martin has set up a Shared Island Unit to strengthen cross-border links.
Jim O’Callaghan, a prominent member of Mr Martin’s party Fianna Fáil, has recently published a document on Irish unity.
His proposals include retaining the Police Service of Northern Ireland (PSNI) after unification and moving one of the two houses of the Irish Parliament to Belfast.
Mr O’Callaghan acknowledges that unity “isn’t a driving political issue” in the Republic.
“This is a highly successful post-colonial independent country and I think people would be hesitant about voting for anything they believe could undermine that,” he says.
“But there’s no reason why that should happen – if there was a good proposal for unity I think people here would support it.”
What are the economic factors?
There is not a firm consensus among academics about the economic outworkings of ending partition.
However, the fact that Dublin-based think tanks are engaging with the issue in a serious way marks a change during recent years.
A century ago, Northern Ireland’s economic strength compared with the rest of the island was a major reason why unionists argued to remain in the UK.
However, its economy now underperforms compared with most other parts of the UK and Ireland.
In the past 40 years, the Republic has been transformed from being one of the poorest countries in Europe to one of the richest.
Mr O’Callaghan says: “Northern Ireland should not have to remain as a poor region of a wealthy country.
“It can and should be a wealthy region of a wealthy country.”
Unionists emphasise that the UK is generally assessed as being the fifth or sixth biggest economy in the world.
Another issue is the subvention – essentially the gap between the sum of money Northern Ireland contributes to the UK Treasury and the amount of public spending in Northern Ireland.
Dr Birnie says figures gathered by the UK Office for National Statistics indicate the deficit is more than £10bn a year.
He argues that endangering the subsidy would be “like unhooking a patient from intensive care”.
Prof John Doyle, from the School of Law at Dublin City University, has recently argued that the subvention which would transfer to a united Ireland would be under £2.5bn and that models of economic growth after unity mean this deficit would be covered.
There is increasing analysis of other issues through the lens of the constitutional question, including healthcare, education and infrastructure.
The Westminster government has schemes designed to strengthen the union.
They include the connectivity review, which is looking at ways to improve transport links across the UK, including a feasibility study for a bridge or tunnel across the Irish Sea.
What lies ahead?
The Northern Ireland Assembly election scheduled for next year and the next UK general election due by 2024 will show if trends are being sustained.
It wants a border poll as soon as possible and would push harder for one as part of any future government coalition deal in Dublin.
However, the decision on whether to call a border poll rests with the British government – and Boris Johnson has said he doesn’t expect a referendum for “a very, very long time”.
Unionist commentator Sarah Creighton says: “At this point I don’t see a united Ireland as being likely.
“But I’m not complacent either.”
The former Taoiseach Bertie Ahern has suggested a border poll should be held on the 30th anniversary of the Good Friday Agreement in 2028.
Mr Carlile from Ireland’s Future thinks it shouldn’t happen any later and so “we should do the preparation now”.
One major difference between how the arguments are being pursued in 2021 – compared with the past – is that violence is not constantly in the background.
The desire to protect peace is something on which almost everyone across the island is already united.
The BBC News NI website has a dedicated section marking the 100th anniversary of the creation of Northern Ireland and partition of the island.
There are special reports on the major figures of the time and the events that shaped modern Ireland available at bbc.co.uk/ni100.
Year ’21: You can also explore how Northern Ireland was created a hundred years ago in the company of Tara Mills and Declan Harvey.
Listen to the latest Year ’21 podcast on BBC Sounds or catch-up on previous episodes.