THEY are the allies Better Together must wish would just shut up.

Northern Ireland's unionists launched another assault on Scottish independence this week with one prominent MP accusing the SNP of "anti-English racism".

Sammy Wilson, of the DUP, reckons he has been caught in the crossfire between Scottish Labour and the nationalists: he sits, after all, between the two warring parties in the House of Commons.

"This is an issue which has the potential to tear the UK apart," he said in a column in Belfast's Newsletter after Alex Salmond and Nicola Sturgeon unveiled their indyplan in Glasgow. "I know the bitterness which has crept into this campaign in which the Scottish Nationalists have become increasingly vitriolic and in some cases racist in their anti-English rhetoric."

Wilson's line in The News Letter would be familiar to Scottish readers: that Salmond's vision was a "myth" because his independence - depending on the EU or London, say, wasn't absolute.

But what interests me about Wilson's intervention isn't its actual content - which is unremarkable. No, what is interesting is the tone of his remarks and the very fact he made them in the first place.

First the tone. Amid fairly bog-standard "too wee, too poor" arguments, Wilson, the corner boy street fighter of the DUP, dismisses Scottish nationalism as a romantic movement.

True, he reckons Scots next year will vote with their heads, not our hearts. Like the people of Northern Ireland, Scots, he believes, know which side their bread is buttered on, and that would be Britain's. "I believe that the Scottish people," he declared, "will see through the mist of this romantic nationalist notion and understand the reality of the choice facing them, just as an increasing number of nationalists in Northern Ireland have done.

"Even 25% of Sinn Fein voters recognise that being within the UK has benefits and only 3% of people would vote to leave the UK tomorrow."

Are Scottish nationalists a bunch of racist romantics? Well, the movement, like any other, has its nutters. But they weren't much in evidence when Salmond and Sturgeon unveiled their painfully dull and painstakingly technocratic blueprint this week.

So why say they are?

Northern Ireland's unionists, including the DUP, may have moved on since Good Friday. But - like it or not - their image is still pretty toxic in Scotland. Many Scots, perhaps unfairly, would instantly brand them as bigots.

The DUP know this. As my colleague Gerry Braiden reported early this year in The Herald, they are staying firmly out of the Scottish debate precisely because of this image problem.

A Belfast loyalist calling a Scottish nationalist an anti-English racist? Not a clever political move this side of the North Channel.

Braiden cited a senior DUP source saying that, despite peacemaker reincarnation of the DUP's Ian Paisley, the idea of his campaigning in the streets of Glasgow was "unlikely to have the desired effect for Better Together".

So why talk now? And why use language like "racist" and "romantic"? Because they have to; because they have to please their core constituencies. Worried unionists across the water needed to hear from their leaders after Salmond and Sturgeon made their big pitch. The media asked for comment. So they gave it - if not always as crudely as Wilson.

Northern Ireland First Minister Peter Robinson used more diplomatic language, according to the Belfast Telegraph. "Nowhere else in the UK would the bonds be more tightly drawn between any other part of the UK from NI's point of view than with Scotland," he said. "Our peoples have moved from one side of that small stretch of water to the other and back many times over the centuries.

"So we have a massive interest and I don't think we can sit idly by and simply indicate that it is a matter for Scotland alone. It will have implications for us all. We hope that Scotland knows just how much we want them to remain within the UK."

Republicans, like unionists, are also staying out of our debate.

The News Letter itself had its say too this week. Much has been made of the existential threat to Irish unionism if Scotland defies the polls and votes yes. But could that other referendum, the one David Cameron has promised on the EU, be even more serious.

"People in Northern Ireland will be watching closely," the paper said in a leader column, " both because of the close historic ties between Scotland and here, and because of the implications for the Province. Those implications will be profound if Scotland leaves the Union."

But the real nightmare scenario? That a rump UK, without the moderating effect of Scottish voters, would go Eurosceptic. That might go down well with some anti-EU unionists in Northern Ireland. But, the theory goes, it would infuriate nationalists across the water and, in the long-run, jeopardise the ongoing British union - and continued subsidies from Westminster. The paper's verdict: "A Scottish exit from a UK that was itself exiting Europe will put considerable pressure on the whole notion of unionism."