Had he not had a history of molestation on his conscience, he would have steadfastly protested his innocence against the lie.
Although the nun has no real proof, she takes his resignation as an admission of guilt.
All through his presidential campaign Sean Gallagher was at pains to portray himself as some kind of community youth worker instead of the Fianna Fáil insider he is.
Despite his recent membership of Fianna Fáil's National Executive and pictures of him rubbing shoulders with Bertie Ahearne, the PR seemed to be working.
However, it seems that someone who understood the true nature and history of the man decided to put the cat among his pigeons.
Since the Broadcasting Authority of Ireland yesterday upheld Sean Gallagher's complaint against RTE, his supporters in the blogosphere have been busy saying that RTE changed the election by broadcasting a lie they received via Twitter.
For those who care to read the BAI's finding itself, it is only the attribution of the Tweet to someone from Martin McGuiness's campaign that is considered erroneous.
The BAI criticises RTE's failure to verify the source despite the fact that the content of the tweet subsequently proved to be true.
"While the Broadcaster states in its response to the complaint that the content of the tweet turned out subsequently to be correct, the Committee did not believe that the subsequent truth or otherwise of the content removed the basic responsibility of the Broadcaster to verify its content and provenance during the programme as it was being broadcast."
Regardless of the allegation itself, it is the source that is deemed to have been false and RTE let itself down badly by not verifying it.
As in the case of Fr. Kevin Reynolds, RTE's journalistic standards were found wanting.
However, unlike Fr Kevin Reynolds, Sean Gallagher did not steadfastly defend his innocence.
Live on national TV, he equivocated and said that he may have collected an envelope.
Yes he was ambushed by the tweeted story on live TV and did not have time to compose himself and put out his usual story.
On mature reflection, the next morning he was back on message but by then it was too late.
His mask had slipped and everyone had seen it.
As far as I am concerned the originator of the mischievous tweet did the country a great service and so did Martin McGuinness and Pat Kenny by putting the allegation to Sean Gallagher.
They gave him the rope. He did the rest himself.
As Sister Aloysius says "Sometimes to right a wrong, you must move away from God".