Brave stand that forced the IRA and Adams into corner
Others had wanted to make a difference. The biggest and most effective protest against the paramilitaries, from within the Catholic community and beyond it, had come from the Peace People.
Thousands across the divide had marched demanding an end to violence following the deaths of the three Maguire children when IRA man Danny Lennon's car careered into them after he was shot at the wheel while on "active service".
But that campaign was powerfully rebutted by the IRA, which refused to take the responsibility alone for making peace.
Thirty years later the IRA was telling us that it wanted peace and was at least prepared to shoulder a share of the burden if others moved too.
Then it killed Robert McCartney.
It was an horrific killing, more in the style of the Shankill Butchers than of the Provos.
Their way was either usually to put a bomb under your car, shoot you on the street, or to detain you and break every bone in your body.
They had a repertoire and stabbing wasn't a familiar part of it.
And such a stabbing, a virtual disembowelling.
Some asked why this killing could become the focus of an international campaign for truth and justice when 3,000 other deaths had failed to rally sufficient outrage. Context was everything. This was 2005. The IRA was under enormous pressure to commit to decommissioning and to allowing devolution to proceed.
Yet, not only had its members murdered and murdered so grotesquely; some had also, just weeks earlier, robbed the Northern Bank of £26 million, subjecting hostages to trauma and terror.
So, when the five McCartney sisters and Robert's partner Bridgeen Hagans demanded justice, people listened. Gerry Adams listened. The IRA Army Council listened. President George Bush and Hillary Clinton listened. Ted Kennedy listened.
Other victims had met the IRA and sought explanations and admissions from them. Most prominent was Gordon Wilson, whose daughter Maire died beside him under the rubble of the Remembrance Day bomb in Enniskillen in 1987 while he held her hand.
He came out of his meeting with the IRA disillusioned and disheartened.
When the McCartneys met it, the IRA was under a kind of pressure to help that it had not felt before. It needed to deny that it had any part, as an organisation, in a pub brawl that turned into slaughter.
It had to protect the highest-ranking IRA man who had been there, the OC Gerard 'Jock' Davison, who was gunned down on a Belfast street on Tuesday.
It had to protect the peace process and it had to protect its own reputation.
We see that in the grandiose statements it made, reminding us all that its volunteers were committed to good conduct.
None of its people would do a 'criminal' thing like that.
And it had to work within the constraint of its policy at that time, which was not to recognise the Police Service of Northern Ireland. Two more recent brave stands against the IRA, from Mairia Cahill and Aine Adams, had to wait until after Sinn Fein accepted the police and permitted members of its community to report crime to them.
All the republican prompts to witnesses of the killing of Robert McCartney, for two years after his death, merely urged people to "come forward" or go to their solicitor.
The understanding in republican communities was still that reporting IRA members to the police could get you killed.
That is the measure of the courage the sisters showed.
The McCartney sisters and Bridgeen Hagans stood up to the IRA before the killing had stopped and at a time when they might as readily have been branded as informers, shot or exiled as treated sympathetically.
And we saw it in how it responded to pressure, conceding by degrees that IRA members had at least participated in the murder and in its public claim that it had offered to shoot the killers.
The IRA had also to protect the reputation of Gerry Adams, preparing to travel to the US for another St Patrick's Day, expecting to be received at the White House.
But Adams was in trouble because nothing could remove the stain of Robert McCartney's blood other than an emphatic end to the IRA campaign, decommissioning and recognition of the police.
It took another two years to achieve all of that, but the need was made plain to Adams at an American Ireland Fund Dinner in Washington attended by the sisters, when Senator John McCain told him plainly that he would have no doors open to him in the US until the criminality stopped.
Catherine, in her book Walls Of Silence, summarised Adams's problem.
"Gerry had ridden two horses for too long," she wrote.
The McCartney sisters, more than anyone, put an end to that.