Interesting new verse there, Dave. I wonder if it might come one of from James N Healy's collections? I've got his Second Book of Irish Ballads here, and though that hasn't got this song, the man's got a knack of having versions of songs like that with extra verses you've never comes across.

It'd fit in as third verse, after the one with Mullingar and before nthe Dublin one, since Athlone is between Mullingar and Dublin.