Surprisingly, amid the many big spending promises made so far in this election campaign, no one has yet pledged to build a bridge between Scotland and Northern Ireland. This boondoggle, popularised by a prime minister who was until recently very keen on the support of the Democratic Unionist party, is conservatively estimated to cost £15bn – though it may be much higher due to the challenge of avoiding second world war munitions dumped in the sea.

The proposal is impractical to the point of impossibility, but the fact that the idea has been talked about at all is revealing. The constitutional fates of Scotland and Northern Ireland are now linked in a way they have never been before. In Northern Ireland, this election campaign has seen the emergence of pro-remain voting alliances – albeit still coloured by traditional green and orange allegiances – and the DUP looks likely to lose at least one of its 10 seats as a result.

But for the constitutional future of Northern Ireland the election results in Scotland are much more important, particularly if Nicola Sturgeon wins an emphatic mandate for a second referendum. Though little discussed in the 2014 referendum – when I worked as a Treasury civil servant on the “no” side – the implications of a successful independence vote in Scotland are enormous on the other side of the Straits of Moyle, the narrow stretch between the Mull of Kintyre and the north Antrim coast. For it is hard to see how Northern Ireland’s union with Britain can survive the end of Scotland’s.

It is true that sentiment on Irish unity has moved in Northern Ireland – largely as a result of Brexit. Moderate Irish nationalists and constitutionally agnostic liberals are, to use the current phrase, having conversations that they were not having a few years ago. Those discussions are tentative – to put it mildly – and constricted by the unspoken rules of a divided society still a long way from healing the wounds of conflict.

In the republic, there is concern over both the divisions in northern society and the fiscal consequences of incorporating a less productive economy heavily reliant on Westminster subsidy. And that is before one even gets to the symbolism of flags and anthems. Because these acknowledged risks exist alongside genuine hopes for what a reconstituted Ireland might look like, even some champions of unity are advocating years of discussion and consultation before a referendum.

This is sensible on its own terms, but it is also predicated on the assumption that the UK’s current constitutional settlement will calmly endure while the island of Ireland considers its options. Any reading of Scottish politics would suggest this assumption is unsafe.

Irish and Scottish fans at the RBS Six Nations match at Croke Park in Dublin. Photograph: Laurence Griffiths/Getty Images

When viewed against the glacial pace of progress in Northern Ireland, change in Scotland has been lightning fast. In the final weeks of Tony Blair’s tenure as prime minister in 2007, two striking things happened that seemed unconnected at the time but were not. First, Blair oversaw the re-establishment of the power-sharing institutions at Stormont, with the leaders of Sinn Féin and the DUP – then Martin McGuinness and Ian Paisley – in charge for the first time, having displaced more moderate parties. Then, less than a fortnight later, the SNP won the largest number of seats in the Scottish parliamentary elections and Alex Salmond became Scotland’s first separatist leader since the Act of Union in 1707.

Not long after becoming first minister, Salmond gave an interview to the Irish Times in which he said unabashedly that independent Ireland inside the EU was a model to nationalists in Scotland – showing what “a small country can achieve, particularly in a European context”. This comparison was more muted during the first referendum in 2014, when Ireland was still dealing with the consequences of its domestic financial crisis, but the Brexit process has made the comparison resonant again. Even a Eurosceptic would have to admit that Ireland’s diplomatic leverage in Brussels has far outweighed the paltry influence which Scottish government ministers have enjoyed in Whitehall during the Brexit process.

In the same 2007 interview, Salmond also said that people of Northern Ireland were to Scotland “the blood of our blood and the bone of our bone”. It is no understatement that Northern Ireland’s distinctiveness, and the reason it is not in the same state as the rest of the island, is in large measure a product of its connection to Scotland. Centuries of two-way migration between Ireland’s north and Scotland’s west have created an odd situation where Scottish saltires are flown in loyalist parts of Ulster and Irish tricolours fly at Celtic football matches in Catholic areas of Glasgow and nearby towns such as Coatbridge. Indeed, a significant chunk of the SNP’s breakthrough in recent years has been due to inroads made into an Irish-descended vote which once went to Scottish Labour en masse. No longer simply the cause of “tartan Tories”, the independence movement claims as its most esteemed academic advocate the doyen of Scottish historians, Tom Devine, who is of working-class Irish descent.

A symbiotic relationship of sorts also exists between the administrations in Dublin and Edinburgh. Ireland both influences the Scottish desire for independence and will be directly affected by it – including in ways it may not yet want. Notwithstanding the willingness of Whitehall – or indeed the English electorate – to continue subsidising Northern Ireland if Scotland leaves the UK, would it be plausible for the people of Belfast to be in the same jurisdiction as those in Bristol, but not Dublin or Glasgow? From the perspectives of ancestry and identity, it would appear to make little sense.

Referendums have a tendency to fool voters into thinking they are addressing only the question on the ballot paper in front of them. Voters in Scotland, both at this election and any new independence referendum, are not only deciding Scotland’s future – but probably Ireland’s too.

• Matthew O’Toole is a former No 10 Brexit spokesperson