Like the sisters of murder victim, Robert McCartney before her, Ms Cahill will not be silenced by the Republican machine.
Ms Cahill’s brave refusal to be just another human piece of collateral damage, swept away with all the rest by the Republican leadership in the all-consuming name of self-preservation, has culminated in a stark moment of truth for Sinn Féin.
But the party has made it clear that in its Troubles-laden mind-set truth remains an elastic concept.
Mr Adams, and his deputy, Mary Lou McDonald, believe Ms Cahill that she was raped by a senior IRA figure.
But it was only after the rape that Ms Cahill started lying, say Mr Adams and Ms McDonald.
They have trashed Ms Cahill’s description of being subjected to a kangaroo court and forced to confront her attacker so that the “judging panel” could “monitor her body language” to test the veracity of her testimony.
Let’s just be grateful the practice was not to throw her in a river and only believe her innocence if she drowned.
In panicked desperation by the party, Sinn Féin TD, Peadar Toibín, attacked the media, as he rounded on radio presenters for their handling of the story, rather than deal with the substance of it.
After laying into Morning Ireland’s Cathal Mac Coille, the TD turned up on Newstalk Breakfast, where he took exception to host Chris Donoghue’s comment that the IRA treatment of a case involving a “volunteer” who raped two teenagers at a safe house in Louth was “mediaeval”.
This is what one of the raped brothers said about what happened, after their family went to the IRA, regarding the crimes, in 2002.
“We were told he could be executed and buried, or be brought to us and we could do him ourselves. The third option was that he would be ordered to leave the country.
“We were traumatised, but we are not murderers, so we decided to go for the third option,” he told a national newspaper. He did not wish to be named, to protect his family.
Mr Toibín did not query the truthfulness of the incident recounted — he merely expressed distaste at the term “medieval” to describe it.
So, two rape victims, who were 14 and 17 at the time of the crimes, are then put through a kangaroo court and, when found to be telling the truth, are offered the choice by Republicans: ‘We’ll murder him for you’; ‘or you can murder him yourselves and we’ll take care of the body’; or ‘we’ll exile him to England’ — where, presumably, he would be free to rape more children.
If that is not mediaeval, then what is?
This is why Sinn Féin has its priorities so wrong, and why its hard-faced stance is so corrosive to itself and those around it.
Mr Tóibín, like Mr Adams, fell back on the old, cold story about how the North had no democratically accountable police service, which meant the IRA was forced to reluctantly administer ‘justice’.
But this was not the turmoil of early 1970s West Belfast under the menacing shadow of the B-Specials.
This was Louth.
This was the Irish Republic.
This was 2002.
And this is finally out in the open, because Ms Cahill would not be cowed.
Her determination has forced Mr Adams to continually retreat, as the ground has shaken beneath him, from talking about “if” her rape allegations were true, to accepting he believed them, to issuing an extraordinary 1,800-word blog on how IRA vigilantes took it upon themselves to “police” republican areas.
Let’s park the fact that Gerry “Army Council? What Army Council?” Adams seems to know an awful lot about the IRA and feels he has the authority to apologise on their behalf, despite the fact, as we all know, he was never a member.
The two most chilling sections noted that: “The IRA on occasion shot alleged sex offenders or expelled them,” and that because the IRA has left the scene “there is no corporate way of verifying” Ms Cahill’s allegations regarding a kangaroo court.
No “corporate” memory, but business as usual, eh, Mr Adams?
And now, the Republicans decry “trial by media” (leaving aside the fact that given the choice I might find trial by media ‘less worse’ than trial by an IRA kangaroo court), there is appalling double-think here from Mr Adams, who went from talking about “if” Ms Cahill was raped to firmly believing she was raped.
How did he suddenly come to that verdict?
And why believe that part of her statement and not the subsequent kangaroo court element?
Mr Adams has urged anyone with information about abusers to go to the gardaí and PSNI.
But the underlying “corporate” message is clear: ‘We’ll believe you — right up until the point it damages Sinn Féin corporately. Then, you’re just another liar.’
And what of the electoral fall-out for Sinn Fein from all this? Judging by past controversies, it will have little impact on the core vote, but will make them even more transfer-toxic than ever.
But this is different to other scandals, as it has more than “something of the North” about it; this one hits home in the South, because we need to know this: where are the paedophiles, rapists and sex offenders the Republicans casually dumped across the border, in the manner that the Catholic Church moved abusers from parish to parish as it facilitated their re-offending?
Mr Adams, given his extraordinary standing in the IRA he never joined, needs to answer the question he evaded in the Dáil, and use his influence to reveal how many sex offenders the Republicans spirited away to the South, and where they are now.
Wednesday’s leaders’ questions in the Dáil were a totemic moment in modern Irish politics that transformed into ‘questions of leadership for Mr Adams’.
Questions that he failed to answer; leadership that he failed to demonstrate.