On September 18, Scotland will hold a referendum on independence. A 'Yes' vote would bring to an end the 1707 Union, leaving behind a rump United Kingdom, comprising England, Wales and a Northern Ireland constitutionally marooned from its nearest – both geographically and emotionally – UK neighbour.
Clearly, Northern Ireland has a lot at stake in September's plebiscite. Nationalists and unionists alike wonder if Scottish independence could have a domino effect, propelling the "break-up of Britain" foreshadowed by Scottish nationalist writer Tom Nairn more than three decades ago leading, eventually, to a United Ireland.
But the response from both sides of the Northern Irish divide to this prospect has been markedly different.
Nationalists have had surprisingly little – at least in public – to say on the subject of Scottish independence. Of course, most would welcome any loosening of the ties that bind the United Kingdom, but they are aware, too, of the politically toxic nature of sectarianism in Scotland.
An overt intervention from Sinn Fein would be political poison for Alex Salmond and the Scottish National Party. So they stand on the sidelines, issue the odd murmur and cross their fingers for a 'Yes' on September 18.
Unionists, on the other hand, seem to be growing restive. Wary of their kin across the Irish Sea flying the constitutional nest, unionists have become increasingly vocal – which is a problem, because their voice is often shrill, their message shallow.
Take Ian Paisley Jnr's comments this week. A 'Yes' vote in Scotland would, the North Antrim MP said, be a spur for dissident republican violence, destabilising Northern Ireland and unravelling the gains of the Good Friday Agreement.
It is unclear how a democratic referendum in another part of the United Kingdom could give succour to gunmen who have minimal support even within their own communities.
If anything, the SNP's success proves beyond a reasonable doubt the supremacy of constitutional means. Regardless of the result in September, Alex Salmond and his party have shown that it is the ballot box – not the Armalite – that works.
Paisley said his comments were designed to awaken supporters of the Union from a complacent stupor arising from the 'No' side's commanding poll lead in Scotland. But his reductio ad absurdum is likely to have the opposite effect, marginalising unionists still further from the heart of the Scottish debate.
In fairness, Paisley is not the only unionist who has tried – and failed – to engage with the question of Scottish independence. In 2012, the Orange Order's Dr David Hume said people of Ulster Scots descent in Northern Ireland should have a vote in September's referendum.
Speaking to a Grand Orange Lodge of Scotland event in Glasgow, Dr Hume said: "We are stakeholders, as well. Surely a decision such as this should not ignore our input?"
But nobody is denying unionists' "input" into the Scottish debate. Extending the franchise to people of Scottish descent in Northern Ireland is completely unworkable – by the same token, what about those in Canada? Australia? England? – but unionists are free to make their case for the Union, to appeal to their Scottish brethren's hearts and heads.
Instead, unionists have pleaded for a vote, or issued thinly-veiled warnings about a return to the Troubles if Scotland decides to go it alone.
Such aggressive tactics are unlikely to prove popular with the Scottish electorate, but, more importantly, they reveal the depth of the existential crisis at the heart of unionism, both in Northern Ireland and across the United Kingdom.
What is the positive case for the Union? This is not a flippant question. Just this week, former Labour Scottish first minister Henry McLeish, writing in The Scotsman, asked why "there is still no sense of urgency about making a positive and modern case for the Union, no sense of grasping the seriousness of the 'Yes' campaign and the impact it is making and no sense of the public disillusionment with Westminster politics?"
That is not to say there is no positive case that unionists can make. Historically, the United Kingdom proved so successful precisely because it is so flexible and capable of reinvention, as Linda Colley reminds us in her new book, Acts of Union and Disunion.
But contemporary unionists seem to be lacking invention. Since the Second World War, unionism has struggled to forge a creative sense of shared identity across these islands.
Meanwhile, the constitutional settlement has come under renewed pressure. Regardless of the result in September, power will continue to seep from Westminster to Edinburgh, Cardiff, Belfast and even, perhaps, a separate English parliament.
The year 2014 is the centenary of another, oft-forgotten constitutional upheaval: the Government of Ireland Act. Even if the First World War had not intervened, unionist resistance might have put paid to Home Rule.
But, 100 years on, unionists now need to develop a compelling, non-coercive case for maintaining the Union. If their attempts to join the Scottish independence debate are anything to go by, they have a long way to go.