Sex abuse scandal could be last straw for Adams ... and hurt Mary Lou's ambitions
In an online interview last week, Mary Lou McDonald was asked if she wants to become Taoiseach one day. The Sinn Fein deputy leader's answer was brief and to the point: "Why not?"
In fact, recent events have shown that there is a very good reason why not. As the IRA child sex abuse scandal gathers pace, Gerry Adams' denial of Mairia Cahill's account is no longer the only issue.
Mary Lou is also showing a total absence of moral leadership - proving once again that the party's transition from terrorists to democrats remains a work in progress.
In fairness to McDonald, she has sounded distinctly rattled in her media outings ever since this story broke. As a woman, she must have been privately horrified to learn that 16-year-old Mairia Cahill was repeatedly raped by a senior member of the IRA back in 1997.
She must also have been deeply disturbed when Mairia and Gerry Adams gave wildly differing accounts of what happened when they met to discuss the matter.
If Sinn Fein was a normal political party, Mary Lou would be fully justified in taking Adams aside and telling him that his credibility has been shot to pieces. Sadly, it is more like a tribe or cult where loyalty is valued over any other principle.
McDonald has already gone on record as saying, "I am absolutely satisfied that Gerry is telling the truth" - which implies that Mairia Cahill is not.
We all know by now that Gerry Adams has no shame. He refused to resign last year when his response to the sex crimes committed by his brother Liam was roundly condemned. He continues to deny any involvement in the murder of mother-of-ten Jean McConville, even though other republicans told a very different story.
Most glaringly of all, the Sinn Fein leader lies with a straight face and treats us like fools every time he claims that he was never a member of the IRA.
If Adams was able to brazen out all those other scandals, there is every chance that he will get away with this one, too. His survival as party leader, however, cannot be the end of the affair.
The details of Mairia Cahill's ordeal are simply too haunting to forget - especially since they fly in the face of the new squeaky-clean Sinn Fein image personified by Mary Lou and others.
At a time when Mairia needed professional care, she was subjected to an IRA kangaroo court that abused her all over again by forcing her to confront the rapist.
The terrified teenager was convinced she would be executed and found herself hoping they would use a gun instead of a baseball bat.
Perhaps the most chilling image is Gerry Adams reportedly telling her that some sex abusers are so manipulative, their victims actually enjoy it - a claim he utterly rejects.
From a PR point of view, Mairia Cahill must be the Shinners' worst nightmare. The young Belfast woman is dignified, articulate and utterly convincing every time she tells her story. As the grand-niece of IRA leader Joe Cahill, she can hardly be accused of being a British stooge either.
All this means that Mary Lou McDonald and her party's younger generation have some hard choices to make. Sinn Fein does well in most opinion polls these days, but it often has trouble translating those numbers into real votes. In the recent Dublin South West by-election, flip-flopping on water charges caused it to be pipped at the post by the Socialist Party. Its support is particularly low among women, possibly because they are more likely to be repulsed by the party's shady past.
Maybe these problems will all disappear on the day that Sinn Fein says, "Well hello Mary Lou, goodbye Gerry." What we know for sure is that every time McDonald has to defend her tainted boss in public, she loses a little bit of her own credibility too.
If Mary Lou really wants to be Taoiseach some day, she must start showing Mairia Cahill some proper compassion - because most Irish people expect more from their leaders than the guff she has offered so far.