David McKittrick: Five angry sisters who have paralysed the IRA

The landscape in Northern Ireland has just altered in the most surprising way, in a manner which - if permanent - is capable of transforming the basic template of politics and the peace process.

The landscape in Northern Ireland has just altered in the most surprising way, in a manner which - if permanent - is capable of transforming the basic template of politics and the peace process.

Nobody saw it coming but everyone is feeling its effects. The public, the politicians and the paramilitaries these days speak of little else, and watch fascinated as it daily sets new rules.

The new feature is the McCartney family from Belfast's Short Strand, which has ignored the tradition of republican omertà and has spoken out about the murder of Robert McCartney.

For weeks it was the Northern Bank robbery that everyone talked about: why the IRA had done it, why they wanted all that money - £26m - and why they would do something that would put all their political gains in jeopardy.

But when Robert McCartney's five angry sisters came forward to blame IRA and Sinn Fein members for the killing, things changed. Republican sympathisers could excuse and even commend a robbery, viewing it as a daring and victimless crime, but this was different.

A Catholic man from a republican district was dead, and though the IRA had not formally sanctioned the killing, its members were involved - two of them high-ranking IRA officers - and IRA methods had been used to dispose of evidence and impose silence.

But the five sisters would not be quiet - one of them estimates that in recent weeks they must have given around 500 television and radio interviews - and interest in the case mushroomed.

In the aftermath of the robbery, the IRA and Sinn Fein response was one of defiant denial, but that would not serve following the McCartney killing. Instead, their approach towards the incident has been characterised by concern and contrition. The IRA has twice called it a "brutal" killing, has expelled three of its members, said people should make statements and promised that no one will be intimidated.

For Sinn Fein, Martin McGuinness said he was "outraged" that IRA volunteers were involved while Gerry Adams is letting the police have the names of seven Sinn Fein members who were in the bar.

In republican terms these are amazing developments, and they are happening because of the McCartney sisters. After each IRA and Sinn Fein move, republicans, and everyone else, turn to the sisters to watch for their reaction. That reaction is generally that they welcome movement but that their clear goal is to have their brother's killers put behind bars. This is a clear goal but it is not a simple one, for convictions will probably require either confessions or eyewitness evidence delivered in open court.

This is going to be hugely difficult to achieve, yet the very simplicity of the demand has turned it into a problem for republicans rather than for the family.

If past experience is anything to go by, there will be many twists and turns to this episode which, together with legal technicalities, could drag on for years.

But already, because the killing was brutal, because the sisters spoke out and because their demand - justice - is so unadorned and so potent, the McCartney family has become a major new moral force.

It is a role that the churches never managed to play in the troubles, for the armed groups paid little or no attention to priests or ministers. The McCartneys are different in that they, like the IRA, are working-class people; indeed - like most IRA leaders - they are highly intelligent and astute people.

They may yet make mistakes which could diminish their standing, but so far they have not put a foot wrong in this challenging campaign, for ever emphasising justice and avoiding political minefields.

For some time London, Dublin and other players in the peace process have been pushing republicans to leave illegality behind and become fully political. The robbery and the pub killing showed that the IRA has so far withstood that pressure.

But now the McCartney campaign has emerged and is placing republicans under almost unprecedented pressure, since those who sympathise with it include grassroots republicans who do not want their cause associated with criminality.

The old template was one of republicans making political progress while surreptitiously indulging in criminality, but the brightest of spotlights is now beamed on this. It is likely to remain there, for the McCartney campaign has arrived to act as a powerful new moral arbiter.