I’ve spent a bit of time looking at marginal constituencies recently, but one I haven’t yet mentioned is the most marginal of them all.

There’s a reason for that. It’s not appeared in any of the lists I’ve compiled of Labour, the Lib Dems or the Tories’ riskiest seats, because it’s not held by any of them. It’s not even in Britain.

It’s the seat of Fermanagh and South Tyrone in Northern Ireland, where Sinn Fein’s Michelle Gildernew has a majority of just four.

This was the result at the last election:

Unsurprisingly, Rodney Connor, the independent candidate in second place, challenged the result. He alleged irregularities in the counting of the votes. But a court ruled that out of the tens of thousands of ballots cast in the contest, only three could not be accounted for. And even if these were all for Connor, they would not change the final outcome and instead merely reduce Gildernew’s majority from four to one. So the result was upheld.

At the election Connor had the support of the DUP and the Ulster Unionists. The two parties had decided to join forces and back an independent in order to avoid the unionist vote splitting, as had happened in 2001 and 2005, which had allowed Sinn Fein to gain what was historically a unionist seat. But their tactic failed – by the tiniest of margins.

From 1983-2001 the MP for Fermanagh and South Tyrone had been the Ulster Unionists’ Ken Maginnis. He won the seat from Owen Carron, the election agent of IRA prisoner Bobby Sands, who himself had won the seat in a by-election in 1981 while behind bars in the Maze. Sands died just 26 days after his victory from a hunger strike. Prior to Sands the seat had been held by independent republican Frank Maguire, and prior to him the seat had been held by… the Ulster Unionists once again.

It’s a remarkable history and something of a microcosm of the course of Northern Ireland politics since the start of the Troubles in the late 1960s.

Fermanagh and South Tyrone could become the focus for drama all over again next year, as even though none of the main Westminster parties are standing in the constituency, were Sinn Fein to hold on it would lower by one the total number of seats either the Tories or Labour would need to win for a notional majority in the Commons.

That’s because Sinn Fein refuses to take its seats at Westminster, hence reducing the total number of MPs at any one time on the opposition benches. Although in theory a party needs 326 or more MPs to form a government, in practice it could command a majority of less than that thanks to Sinn Fein’s absence*.

Such calculations could prove crucial next year were either the Tories or Labour to fall short of 326 by half a dozen or so MPs.

Equally significant would be a gain in Fermanagh and South Tyrone for a Unionist candidate, who might then be willing to vote with a Tory minority government.

Such ponderables are all part and parcel of this constituency’s reputation for delivering unexpected outcomes that can lead to equally unexpected consequences.

In an extreme scenario, the identity of the next government, never mind that of the next MP for Fermanagh and South Tyrone, could come down to a handful votes.

*Andrew has left a comment below that elaborates on this point rather well.