"Soldiers of the Queen"--intended as irony...
"Soldiers of the Queen" is sung by Breaker Morant at the end of the movie. The movie essentially lays out criminal behavior Breaker Morant engaged in while being a 'soldier of the Queen'. Except once he, or any soldier, engages in criminal activity, he is no longer a 'soldier of the Queen'; he is a criminal. That is the whole point of a court martial--to determine the nature of the crime, if one had been committed, and mete out the proper punishment.
Breaker Morant, does not see himself as a criminal, but sees himself as a 'soldier of the Queen" doing his duty as a soldier of the Queen.
But the movie makes for a compelling case, based on reality of the event, for the audience to see Breaker Morant engaging in criminal behavior.
So while Breaker Morant does not see the irony in the song, the audience is expected to see that ironic overtone.
(And an important parenthetical observation: the title of the movie--Breaker Morant--wants to place emphasis on the ego of someone named Breaker Morant. The title is not "Soldiers of the Queen" or "Scapegoats of the Empire". It's like the director, the playwright, wanted to emphasis the ego of Breaker Morant and not the principle of soldier as implementer of the Queen's policy.)