If there's one thing that really gets an ardent Brexiteer going, it's the thought of a post-Brexit US-UK trade deal.

"President Trump has made clear again that he wants an ambitious free trade agreement with the UK," UK Foreign Secretary Dominic Raab said lustily earlier this month, during a visit to Washington. "So I hope we can make that happen as soon as possible after we leave the EU on 31 October."

And, if there is something hard Brexit supporters like Raab hate with the same ferocity as they love such a trade deal, it's the so-called "Irish backstop".

That's the insurance policy provision agreed by both sides in the EU-UK Withdrawal Agreement which would avoid the restoration of a hard border between Ireland and Northern Ireland until such time as a prospective UK-EU free trade deal would achieve the same.

Now imagine the following scenario: The UK leaves the EU without a deal on October 31, something that looks likelier by the day. Amid the ensuing chaos, it manages to do a trade deal with the US in record time, to try and mitigate some of the damage done by the unravelling of its most important economic relationship – that with the rest of the EU.

Yet, despite the fact that US and UK negotiators have somehow managed to whip up a trade deal quickly (highly unlikely, given the nature of trade deal negotiation), the deal quickly finds itself blocked indefinitely by a furious US Congress.

And why? Because of the Irish border. It might sound like Raab's worst nightmare, but the prospect is more real than he would care to imagine.

Another special relationship

Irish-American political power has caught Britain off guard before. Over a century ago, the influence of Americans with Irish origins proved critical to Ireland winning its independence from British rule.

Then, when neither the British nor Irish governments could contain the sectarian violence which ripped apart Northern Ireland for 30 yearsfrom 1968-1998, an American intervention proved decisive.

In the end it was the proactive approach of the Clinton administration, driven by prominent Irish-American politicians such as Edward Kennedy, which did more than anyone to end the violence.

The American role in bringing peace to Northern Ireland is widely seen as the landmark achievement of Irish-American political influence, and it's for that reason that the US Congress could block any US-UK trade deal which could arise in the event of a UK exit.

In 2011, then US President Barack Obama visited the village of Moneygall in Co Offaly, Ireland - the ancestral home of his great-great-great grandfather. It was a nod to the enduring power of the so-called "green vote" in the US

No deal – and no trade deal either

Speaker of the US House of Representatives Nancy Pelosi is arguably the second-most powerful person in Washington, after President Trump.

She has been unambiguous in how she would approach a UK-US trade deal in the event of a British exit which changes the current circumstances on the island of Ireland.

"We must ensure that nothing happens in the Brexit discussions that imperils the Good Friday accord," she told the Irish parliament following a visit to the border city of Derry in April of this year.

She said there was "no chance" of a post-Brexit UK-US trade deal if the Northern Ireland border became anything other than seamless as a result of the UK leaving the EU.

Given the sway she holds over Democratic voting, it's almost impossible to see how a US-UK trade deal could be voted through without her support.

She's far from a lone voice. The influential Friends of Ireland caucus is a bipartisan group of 54 senators and representatives which has repeatedly said it will not support a UK-US trade deal if the Northern Ireland peace agreement is undermined by the return of a hard border.

Nancy Pelosi visited the border city of Derry in April and said there was "no chance" of a US-UK trade deal if the Irish border becomes anything other than seamless

The group wrote to new British Prime Minister Boris Johnson recently, telling him:

"We will oppose any US-UK trade deal if the Good Friday Agreement is undermined. As you know, America is guarantor of that international peace accord. That is why we strongly oppose any unraveling of the historic treaty or a return of a physical border on the island of Ireland under any circumstances."

Tricks of the trade deal

The Democratic co-chair of that grouping, Richard Neal, told The Guardian last month that he would "have little enthusiasm for entertaining a bilateral trade agreement with the UK, if they were to jeopardize the agreement."

As chair of the US House Committee on Ways and Means, Neal will lead the scrutiny of any trade deals which come before it.

That leads on to a crucial point. Although the Trump presidency has somewhat undermined the authority of the US Congress, one area where it retains significant power is on the subject of trade deals.

Trade deals need majority support in both houses of Congress, but they can be blocked for indefinite periods of time as well. In 2008, Nancy Pelosi and a Democratic-controlled Congress refused to back a US-Colombia trade deal backed by then President George W. Bush.

Many in the US and British governments, such as Dominic Raab (left) and Mike Pompeo (right) have spoken enthusiastically about a US-UK trade deal

Using a mechanism whereby votes on trade deals can be held up, Pelosi's opposition ensured the Colombia trade deal was blocked for several years.

Right now, the power of Congress in blocking trade legislation can be seen with the revised NAFTA deal, known as USMCA.It is relatively controversy-free, yet it has been held up in Congress for close to a year now.

Trying to win friends and influence people

It's important to point out that Ireland is not alone in having friends in high places on Capitol Hill.

Earlier this month, Arkansas Senator Tom Cotton and 44 of his Republican colleagues sent a letter of support to Boris Johnson, pledging unspecified support for the UK in the event of a no-deal Brexit.

Then there's the president himself, who speaks as enthusiastically as anyone about a UK-US trade deal. "We're going to do a very big trade deal, bigger than we've ever had with the UK," he said at the G7 summit on the weekend.

But as things stand, Cotton would not have the numbers to beat Democratic opposition. And that doesn't even account for the significant opposition within the Republican Party, many of whom fully back the Irish position on the backstop.

As for Trump, even though he might try, he does not have the power to push trade agreements through without the assent of Congress.

The Irish border (pictured, Ireland on the left, Northern Ireland on the right) could cast a shadow over the UK's Brexit project for decades.

Back to the dreary steeples

Brexit supporters in the UK government have consistently played down concerns over the Irish border, saying they would not construct border infrastructure after Brexit and also suggesting that it would not even be necessary.

But after a no-deal Brexit (which would mean no backstop) an open border in Ireland would mean the European Single Market was compromised. That is something the EU would find intolerable and Ireland, as a member, would need to resolve the issue as soon as possible.

The UK may carry on regardless, hoping that a quick deal with the US would be the first step on the road to recovery after the first waves of the no-deal chaos have been felt.

But it's at that very point that a new reality may dawn: that there was another even more special relationship than the one they pinned their hopes on.