Behan was drunk when he fired at the guard (policeman). He was lucky that he missed for the Free State government of the day would probably have executed him if he had kiled the guard.

For the reasons for questioning Behan's authorship of the song, see Michael O'Sullivan book 'Brendan Behan: a life', described by Seumas Behan as 'the definitive biography of my brother'. Probably, somebody else wrote the song. Behan didn't originally claim authorship, he just kind grew into it when the real author disappeared off the scene, dead or in jail. The song was featured in 'Quare Fellow' (sung from the wings by Behan himself on opening night) but it wasn't written for the play, being years older.