Yesterday’s statement by deputy PM Nick Clegg suggests that Northern Ireland will not have 18 seats in the 2015 Westminster election. The policy of reducing the House of Commons to 600 MPs, all of whom (apart from two in Scotland) will represent constituencies of almost equal sizes, would indicate that Northern Ireland, with about 1/39 of the UK’s voters, would end up with 1/39 of 600 MPs, which is to say about 15.4 – and since you can’t have 0.4 of an MP, that means 15 seats in Northern Ireland, down from the current 18.

The current 18 constituencies, ranked in order of electorate (using the figures for 1 July 2010) are:

59,612 East Belfast
60,256 South Belfast
60,287 West Belfast
61,063 East Antrim
61,181 Strangford
61,230 North Down
62,045 West Tyrone
63,972 South Antrim
64,139 East Londonderry
65,453 Mid Ulster
66,132 Lagan Valley
66,597 North Belfast
67,161 Foyle
68,926 Fermanagh and South Tyrone
71,937 South Down
74,181 North Antrim
75,481 Newry and Armagh
75,773 Upper Bann

That’s a total of 1,185,426; divided by 15 that comes out close enough to 79,000 voters, give or take, in each new seat. The margin of variation is supposed to be 5% either way; if that’s taken as a strict guideline for the whole UK, it’s going to be a very tight constraint on drawing the Northern Irish seats, as the upper bound for any UK seat will thus be around 79,500. More likely, I’d have thought, that they will allow the pragmatic option of 5% variation for NI seats around the local NI average, which therefore means seat sizes between 75,000 and 83,000 will be acceptable, so I shall take that as a working assumption below.

The first strategic question for the boundary revision will be how many seats are needed to cover Belfast. Adding the four current seats together gives enough voters for 3.12 seats in the new dispensation, so one option will be to simply use the current boundaries, maybe trimmed a little, and cut the city to three seats rather than its historic four. When a previous boundary commission in the mid-1990s tried this, it was met with howls of anguish, and instead the four Belfast seats were padded with a few more voters to bring them up to the bare minimum, an approach also taken by the most recent boundary commission.

But to retain four seats this time, about 12,500 voters would need to be added to North Belfast and about 15,000 to the other three seats. The Belfast seats have already extended to include pretty much anything that can reasonably be defined as Greater Belfast, and a bit beyond. To push the boundaries out further could mean East Belfast absorbing Newtownards, thus gaining a shoreline on Strangford Lough; while West Belfast might similarly have to extend as far as Lough Neagh. I must say it’s quite difficult to see this.

So let’s consider the three seat option. This would leave the external parliamentary boundaries of Belfast largely unchanged, allowing more freedom for tinkering elsewhere. Given that Belfast Lough is a fairly unavoidable geographical feature, East Belfast would necessarily annex about a third of South Belfast, North Belfast taking about a fifth of West Belfast, and the remnant forming a new South-West Belfast constituency, which was what was proposed in the mid-1990s. I can’t see this being too popular, as the burghers of the Malone Road wake up to the probability of Gerry Adams being their new MP.

There is another option. The whole of Belfast City Council – now preserved by the procrastination over local government reform – has only enough voters for 2.15 seats under the new dispensation. What price "North and West Belfast" and "East and South Belfast" as the two new constituencies, the hinterlands hived off to the suburbs? The former would certainly be a Sinn Fein seat, the DUP losing Nigel Dodds; the latter could be a close call between the DUP and Alliance, with the SDLP playing an interesting role.

Elsewhere it’s easier to see how the cat might be skinned. Newry & Armagh and Upper Bann are both over 95% of the new quota anyway, so I’m inclined to think they won’t be changed at all, having survived the last boundary commission unscathed, and if so they make a convenient anchor around which to construct the rest of my assumptions.

In the three western historic counties, there are currently five seats (Foyle, East Londonderry, West Tyrone, Mid Ulster, and Fermanagh-South Tyrone) whose combined electorate gives you 4.15 seats under the new dispensation. The two northern seats will expand southward, FST will annex some more of Tyrone, and the amalgamated remnants of West Tyrone and Mid Ulster will be the final seat in the west.

The three County Antrim constituencies between them have enough voters for 2.52 seats under the proposed new system. Again, a lot will depend on what is done strategically with Belfast, but it seems inevitable that North Antrim will reabsorb the areas it lost to East Antrim in the most recent revision which brings it up to 78,000, close to the new avergae of 79,000. A South Antrim seat including almost all of Antrim, Larne and Carrickfergus would have an electorate of almost 84,000, which is just above the 83,000 upper limit (and one can easily see marginal adjustments which would bring it down). That leaves Newtownabbey, and whatever happens to the Belfast seats. One can imagine (I think that someone actually proposed) a massive shift of North Belfast to take up most if not all of Newtownabbey, shedding the rest to West Belfast.

South Down probably regains some of the territory around Strangford Lough that it lost in the last revision, but the other County Down seats depend on what strategic decision is made in Belfast. Strangford as a seat has been dreadfully messed with at every boundary revision since it was created, and this time it will probably disappear entirely between a much bigger North Down, an expanded Lagan Valley, and whatever is done with the Belfast seats.

It is tricky to call winners and losers, but simply because the DUP and SF have the most seats, they are also most likely to lose overall – the DUP probably will lose two in the east, and SF one in the west. As I said just after the election, I don’t think AV would deliver much different results to FPTP, if the DUP and SF continue to maintain their dominance on their respective sides of the community. Of course, it does meant that the tipping points are in different places, and the fact that tactical voting is unnecessary with AV may also change voting patterns.

The proposed new division, if it is as equal as we are promised, will at least take care of one of my persistent criticisms of the current electoral set-up, which is that Assembly seats should be better mapped to electorate – East Belfast, South Belfast and West Belfast do not deserve a sixth Assembly seat on current figures; South Down, North Antrim, Newry & Armagh and Upper Bann all deserve a seventh. But if the variation of 5% is adhered to as a rigid limit, that problem at least will be solved, even if many more are created.