Many years ago, and mainly because of the way it looked, I bought a book called Handbook of the Ulster Question, which was published by the Stationery Office, Dublin, in 1923. It has a green cardboard binding and a front cover decorated in the Celtic revival style, which the Irish Free State also adopted for its postage stamps.

An empty wallet inside the back cover once contained maps. “Lacks maps,” says a pencilled note on the front endpaper next to its price (30p), though the text pages still have a few that fold out, including one of Ireland’s then extensive railway system, which has a dotted line crossing it in 19 places. The dotted line represents the border created by the partition of Ireland in 1921. Suddenly, we are in modern territory. The map shows how “the customs land frontier of the Six-County Area [of Ulster]” will interfere “with the normal traffic of the country” to poor effect.

The line separates Londonderry from its Donegal hinterland and the town of Newry from its port, Greenore. It twists back and forth across one of the cross-country routes of the largest railway, the Great Northern, no fewer than six times in seven miles. (Forty years later, very little of this railway remained apart from its main line from Dublin to Belfast. Its tunnels grew mushrooms. A withering of branches to border towns such as Clones and Enniskillen left middle Ireland train-less. Of course, there was more to blame than the border – railways closed everywhere – but the rot started with partition.)

Some think 20 years of relative peace may have broken the link between political failure and blood on the streets

“As Ireland is an obvious geographical unit, so also it is an obvious economic unit,” says the handbook’s introduction, “and it is in the economic field … that the evils of partition become most apparent.” As a customs frontier would be “an inevitable and automatic consequence”, the introduction continues, not even the most enthusiastic supporter of partition would defend it on economic grounds: “There, beyond all denial, it stands self-condemned.”

In 1923 the border was still provisional. The UK government had appointed a boundary commission that would decide its final course after reading submissions of evidence from the two sides. My “handbook” is really a compilation of the Dublin government’s case, which very roughly was that any partition at all was a bad idea (“calamitous in practice”), but that its more glaring injustices could be corrected “by restoring the maximum number of unwilling citizens to the government of their choice”.

What this meant was drawing the boundaries of the new northern state more tightly, to exclude Catholic-majority areas such as Derry City and the counties of Fermanagh and Tyrone. Many tables and charts support Dublin’s case in statistical detail; scholarly analogies are made with the boundary set between Germany and Poland in Upper Silesia, and the “mixed population” of Transylvania. But the boundary commission paid no notice to any of this evidence, and in 1925 simply confirmed the border as it already existed – in the handbook’s words, “an arbitrary line in a small country clearly marked for unity by the sea”.

For the next 70-odd years, the sea was no match for the roar of political and religious identities. And then, in 1998, came the Belfast or Good Friday agreement, a multilayered and subtle set of documents that mandated a power-sharing government for Northern Ireland, and, as an international agreement, redefined the triangular relationship between Belfast, Dublin and London. Thanks to it, the border was minimised as a contentious physical and psychological presence in Irish life. The army’s watchtowers and the queues of lorries at frontier posts disappeared. Now only the keenest-eyed travellers know when they have left one country for another.

The agreement was popular. Separate referendums in the Republic and Northern Ireland endorsed its implementation by votes of 94% and 71% respectively, and last year the United Kingdom agreed with the European Union that it would protect all the agreement’s provisions after Brexit. We all know the difficulty here. Maintaining a “frictionless” border means that the UK must keep its regulations fully aligned to those of the internal market and the customs union, which seems impossible unless the British government modifies its stated ambition to leave both. Or unless the credibility of the Good Friday agreement can be undermined to the point where it needs revision and replacement.

In this context, last week’s attack on the agreement by a trio of Brexiteers is a development worth unpicking. On 15 February, the Irish historian Ruth Dudley Edwards wrote a Daily Telegraph article on the 13-month impasse at Stormont that concluded: “Realists believe the [deal] has served its purpose and run its course, leaving behind the unintended consequence of enshrining sectarianism in the political process.”

The following day the Tory MP Owen Paterson, a former Northern Ireland secretary, tweeted a link to the piece with a comment: “The collapse of power-sharing in Northern Ireland shows the Good Friday agreement has outlived its use.” Last Saturday the Tory MEP Daniel Hannan wrote, again in the Telegraph, that the agreement was “often spoken about in quasi-religious terms … but its flaws have become clearer over time”. Finally, two days later, the Labour MP Kate Hoey told the Huffington Post that she thought the agreement needed “a cold, rational look”, and that the power-sharing Northern Ireland executive it mandated was “not sustainable in the long term”.

The condemnation of Paterson, Hannan and Hoey has been fierce: this week Hoey told the Northern Ireland affairs select committee that finding fault with the Good Friday agreement was “as if you’re saying you want to kill all babies at birth”. In their defence, all three could argue their criticism was directed at Stormont’s failure rather than the agreement’s international aspects, such as cross-border trade and cooperation. But their long-held position on Brexit – Hannan was one of its chief architects – suggests a different motive: “If the Belfast agreement must die so that the glorious ideal of Brexit may live, so be it,” to quote Fintan O’Toole in today’s Irish Times about this “cynical and reckless” exercise.

Where did I pay my 30p for the Handbook to the Ulster Question? I think in Belfast in the early 1970s: a secondhand shop somewhere near City Hall. The owner, on hearing the thud of a distant IRA bomb, said nonchalantly, “Oh, I thought it was time we had one of those today.” There were many old books on theology; helmeted British soldiers in the streets; rain.

Many people fear that a revival of a hard border and the abandonment of the Good Friday agreement will mean a return to those days. Enemies of the agreement, who include members of the Democratic Unionist party, describe this as moral blackmail; others (including O’Toole) think that 20 years of relative peace may at last have broken the link between political failure and blood on the streets. But say the worst happened – would the British army return to enforce the law? Would its soldiers be willing to risk life and limb knowing it was the perversity and vanity of their politicians that had put them there – a final British misadventure? There would be no stomach for it.