Dominic Lawson: Why should Democratic Unionists hold us to ransom?

When Westminster votes can be won only with the aid of MPs from Northern Ireland, it is a sure sign of a Government in decay. It was true of the Callaghan years – and of the fag-end of John Major's administration.

The nine Democratic Unionist votes for Mr Brown's benighted 42-day pre-charge detention were the exact extent of the difference between the Noes and the Ayes – if they had abstained, the measure would have required the Speaker's casting vote. Had they voted "no" – as they did against Mr Blair's plans for 90-day pre-charge detention – then nothing could have saved the Bill.

Let us take at face value the DUP leader's insistence that his party's last-minute decision to back Mr Brown was not based on any deal, although the broad grin on Peter Robinson's face – not its normal configuration – seemed designed to suggest the opposite. Let us suppose that it was Mr Brown's charm and winning ways which persuaded the DUP. Let us accept that it will be a mere coincidence should Mr Robinson be appointed a member of the House of Commons Security and Intelligence Committee.

As for the large sums for DUP pet projects that furious Conservatives claim have been extorted as the price of their support – well, that would be a disgrace, if true, even if it represents the merest drop in the sea of subsidy that has long swept from Westminster into Northern Ireland. In any case, no hidden deal could have been more shamefully embarrassing than that openly conceded as the cost of getting the vote of Mohammad Sarwar, the Muslim MP for Glasgow Central: the provision that any detainee held for more than 28 days and subsequently released without charge would be eligible for financial compensation. This was an official admission that the proposed new law has injustice at its heart.

Glasgow Airport, of course, was the site of a botched Islamist terrorist attack last summer: it was Mr Brown's steady response to that which won him so many plaudits in the first days of his Premiership, a lost national mood which his latest counter-terrorist measure seems – pathetically – designed to revive.

The Democratic Unionists proclaim that it is their great insight into the nature of terrorism which led them – all of a sudden, on principle – to back Mr Brown. Their view would not be shared by British intelligence officers, who look back on the introduction of indefinite internment in Northern Ireland (admittedly a much harsher measure) as disastrously counter-productive in the fight against terrorism.

Northern Ireland is hardly in the front line in the fight against Islamist terror – I have not heard it said that Belfast is a hotbed of al-Qa'ida cells. On the other hand, a significant proportion of those on the Labour benches who voted against the measure represent London constituencies: "rebel" MPs such as Kate Hoey, Frank Dobson and Diane Abbott need no lectures from across the Irish Sea on the risks to constituents from the home-grown jihadist movement.

Not the least of the ironies in this sordid episode is that last month the Stormont assembly was to have taken control of the province's policing and justice – the final link in the chain of self-government for Northern Ireland. It is the DUP who have held this up – presumably based on residual concerns about the counter-terrorist credentials of their Sinn Fein partners.

So far, in this particular area of government, we are not yet in a "West Lothian" predicament: that is to say, where those elected to Westminster from Scotland can vote in the Commons on purely "English" matters, while English MPs are (naturally) ineligible to vote on similar issues in so far as they affect the devolved region. Yet it was in respect of Ireland, rather than Scotland, that this imbalance was first identified as a fundamental democratic problem. It came to the fore during the debates over Gladstone's proposals for Irish Home Rule – a century before the anti-devolutionist West Lothian MP Tam Dalyell raised the matter.

The DUP are a fine example of that modern parliamentary monstrosity, so-called "Unionists" who enjoy all the trappings (and powers, and perks) of a devolved assembly and who have a fiercely parochial view of politics. Yet they have also shown no hesitation in voting at Westminster on legislation which does not affect Northern Ireland; this tendency, on the part of Scottish MPs, had already infuriated Conservatives, most notably when the Scots at Westminster voted en bloc to ban fox-hunting in England, the practice having already been banned in Scotland by its own parliament.

Next month David Cameron is expected to endorse plans drawn up by Ken Clarke, the former chancellor, which propose that legislation applying just to England should only be voted on by MPs representing English seats. Hitherto, the Conservatives had been much less exercised by the "double" influence of Northern Ireland MPs – the Labour Party has no representation in that province; but after this week's events, I imagine that Mr Cameron will show not the slightest sympathy for any DUP objections to Mr Clarke's proposals.

After the vote on Wednesday, the DUP MP Gregory Campbell gloated over the increased influence that his party would enjoy, as Gordon Brown's parliamentary authority waned: in which case, expect an even stronger stench of decay and double-dealing to emanate from the Palace of Westminster.