Speed 4

In The Passion of Saint Tibilus, we are given a quick run down for why each of the three priests are on Craggy Island. Ted has the Lourdes incident which is brought up repeatedly throughout the series, Dougal is implied to have irreparably damaged a large number of lives, a good chunk of which were nuns, at Blackrock (which may have had something to do with a Sealink Ferry), but Jack’s reason for being on Craggy Island is the most obscure. The most we have to go off of is that he botched a wedding in Athlone, which he remembers fondly, given his shit-eating grin at hearing it brought up by Bishop Brennan.
Apart from that, no specifics on the extent on Jack’s actions are given, so what are your interpretations on what happened at the Athlone wedding?