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Constance Markievicz, thou should'st be living at this hour.

The "rebel countess" was the first woman to be elected to the Commons, in 1918 - but as a Sinn Fein candidate she didn't take her seat and became a TD in the first Dail, instead. Hence the careful wording of pub quiz questions about the first woman MP, which usually include some variation of the phrase "to sit in the Commons".

But with chatter suggesting Sinn Fein might abandon its century old abstentionist policy of contesting elections to the UK Parliament, but then refusing to take up its seats, I fell to wondering what the effect might be if the party did become active players in Westminster. Because it would change the arithmetic of the current Commons in quite a significant way.

Take the current situation. The government majority is boosted slightly by the absence of the Sinn Fein MPs - if they were to engage in Westminster, David Cameron's theoretical majority would dip to 78. Still a pretty comfortable number, you might think, but things become pretty interesting if the Conservatives and the 57 Lib Dems are voting on opposite sides.

A vote that pitted the Conservatives against Labour plus the Lib Dems would be decided by the galaxy of smaller parties - the eight DUP MPs, the six SNP MPs, the three from Plaid Cymru, the three from the SDLP, the various one-person parties like Respect, the Greens and the Alliance - and into that mix we might be able to add the four Sinn Fein members (with Martin McGuiness having resigned they're defending his now-vacant Mid Ulster seat).

And we do have an issue over which this division might happen.

The Lib Dems have withdrawn their support from the proposed change to parliamentary boundaries, which would have cut the number of MPs in the Commons from 650 to 600, by creating fewer, larger constituencies. The Conservatives regard the existing pattern of seats as skewed against them, and changing that bias would be an important boost to their hopes at the next election - but after the defeat of Lords reform at the hands of Tory backbenchers, Nick Clegg and his MPs will now vote against, in revenge.

It remains theoretically possible for the Conservatives to corral the smaller parties into their lobby and force the changes through - but the by-election defeat in Corby, in the wake of the departure from the Commons of Louise Mensch, has made the arithmetic a little tougher. Add a Sinn Fein contingent into this kind of situation, and suddenly there is another small party which might be wooed by a policy concession here, and a bit of extra spending there.

It seems unlikely that the Sinn Fein position would change in this Parliament - were they to take up their seats, they'd probably want to say so to their voters in a manifesto, first. And there would have to be some deal on the form of the oath MPs take to the Queen... but that would probably not be an insuperable obstacle.

Then they'd be a factor in what might be a very finely balanced future parliament. If they could send a solid bloc of five or more MPs to Westminster, they could exert some considerable leverage. Maybe not so much over the great sweep of national policy, but quite probably over spending decisions affecting Northern Ireland in general, and their constituencies in particular. But it would take very advanced wheeler-dealing skills to make concessions to Sinn Fein which didn't alienate the DUP and maybe other Ulster parties....back in 1979, one reason for the fall of Jim Callaghan's minority Labour government was that concessions to the Ulster Unionists had alienated the SDLP's Gerry Fitt, who had been a natural Labour supporter.

But if hung parliaments become the norm, for a while, there would clearly be an opportunity to capitalise on their electoral strength in Northern Ireland. Straddling three parliaments, in London, Dublin and Stormont, without getting tangled up in the contradictions of three different political environments, would take a certain agility... but it could bring considerable rewards.

If they ever brought themselves to change that ancient and ingrained commandment to shun Westminster.