Unsurprisingly, Northern Ireland’s unionists have never exactly welcomed the prospect of Scottish independence. In 2012, then Ulster Unionist leader Tom Elliott descried the Scottish National Party as “a greater threat to the union than the violence of the IRA”.

Earlier this year, DUP dauphin Ian Paisley jnr said that independence on the other side of the Sea of Moyle would give succour to dissident republicans in Belfast. Such shrill prognostications are all too common among unionist voices but they are worth returning to in the wake of the Yes campaign’s slender lead in polls published last weekend.

Support for the pro-UK side has slipped an eye-watering 24 points in just one month. Unionists in Ulster have talked tough in public about Scotland leaving the union, but the reality is that in private few, if any, has actually considered this a realistic possibility.

David Trimble told me earlier this year that Scotland would vote “two to one” to stay in the UK. There been little or no scenario-planning in Stormont about what Scottish independence might mean for Northern Ireland. This will have to change – and fast.

Whatever happens when Scots go to the polls on September 18th, it is becoming increasingly apparent that the UK’s constitutional arrangement is changing, with potential significant repercussions in Northern Ireland.

Protestants in Ulster have long celebrated their links with Scotland. Orange Order leaders in the North recently renewed their calls for Scots to reject independence: this Saturday, thousands of Northern Irish Orangemen are expected to descend on Edinburgh for a massive Orange parade. (The march is causing consternation among some pro-UK campaigners who worry it could alienate Scots of Catholic Irish descent.)

While Northern Orangemen are busy parading alongside their brethren across the North Channel, unionists leaders in Ireland would do well to consider the impact of the Scottish debate on their political settlement.

In an attempt to stave off the threat of independence, a united front of Westminster leaders is due to offer Edinburgh enhanced devolution – the so-called “devo max” option left off the referendum ballot paper – in the coming days. What this devolution package will look like is unclear, but Welsh first minister Carwyn Jones has already declared that any new powers for Scotland should also be offered to Wales and Northern Ireland.

Stormont’s block grant

More levers, including a measure of fiscal autonomy, would almost certainly result in a reduction in Stormont’s block grant. The capacity for budget wrangling to paralyse a chronically weak Northern Ireland assembly has been in evidence this summer. Proposed cuts to the health service have caused more political problems between Sinn Féin and the DUP than any number of flags or parades.

Less cash to spread around the various orange and green fiefdoms in future could create very real difficulties for this power-sharing government in Belfast. This, more than republican dreams of a Border poll, could lead to serious destabilisation – a point not lost on Sinn Féin, who cannily have keep a stony silence on the Scottish question. Independence for Scotland would be no windfall for anti- Belfast Agreement republicans – the SNP has shown what can be achieved by exclusively peaceful means.

But Scotland’s departure would leave the North even more unmoored from a Westminster dominated by right-wing metropolitan Tories, such as David Cameron and George Osborne, who came of age in the 1980s and have little time for and even less interest in “the dreary steeples of Fermanagh and Tyrone”.

Of course such fears can be overplayed. The No side, Better Together, is still odds-on favourite to crawl across the finish line next week. For almost two years, pro-UK voices have argued fiercely against Scottish independence, issuing increasingly doom-laden warning about everything from currency and jobs to European Union membership.

While “Project Fear” has scared sections of the body politic rigid, an emotional case for the union – a reason why Britain really is “Better Together” – has been conspicuous by its absence.

The referendum campaign in Scotland has reduced the benefits of union down to pounds, shillings and pence. Such instrumental unionism (“vote No or you’ll be worse off”) could well win the day next week, but it bodes ill for a harmonious “United Kingdom” – and even worse for the kind of non-sectarian, Danny Boyle-inspired “21st century unionism” that the more enlightened in the North have long talked of developing.

Unionism in Britain has long meant different things in different places. In Scotland it always had a transactional core. The 1707 union between England and Scotland was a compromise. Scotland retained its own systems of law and local government, its own parish schools and universities, its own forms of church government.

Sense of itself

Scotland held on to a sense of itself as a separate country within a union that it had willingly joined and which it measured the success of almost singularly in material gain. Without this distinctiveness there would, almost certainly, be no referendum. The situation in Ireland was very different. Most Catholics felt minimal attachment to the British state and, as the Famine illustrated, the British state felt minimal attachment to them.

Those who felt most buttressed in the newly coined United Kingdom after the 1801 union were Protestants, particularly in Ulster. For them, the union was an existential as well as a material boon.

Whatever the result, the referendum will have a real impact on what the United Kingdom is and how it operates. For unionist leaders in Belfast, the fear must be that even if the relentlessly negative pro-UK campaign is successful in Scotland it might end up curing the nationalist disease but killing the unionist patient. Peter Geoghegan is an Irish writer and journalist based in Glasgow