Ben Hall was "a peaceful, quiet man until he met Sir Fred." Then, with his homestead burnt and his cattle dead, he turned outlaw. The song describes the reward for Dunn, Gilbert, and Ben, and exhorts the listeners to toast their memories
On the basis of internal references (see below), this song might be a variant of "Ben Hall." However, the metre is slightly different and there are few similarities of texts beyond the names of the robbers.
Ben Hall is widely regarded as "the noblest of the bushrangers." This song tells the common story that he was hounded from his home by the police, and only then turned to crime. Even as a bushranger, he attacked only the rich and never shed blood.
The truth is not quite so pretty; for background, see the notes to "Ben Hall."
Dunn and Gilbert, like Hall, were associated with Frank Gardiner's outlaw band. John Gilbert brought the full force of the law down on the gang when he shot a policeman, and he died along with Johnny Dunn in 1866. Johnny O'Meally, also mentioned in the song, was a member of the gang killed in 1863. (Gardiner was eventually taken, but was paroled after ten years and allowed to emigrate to the U.S., where he opened a saloon and, it is said, was shot in a poker fight in 1903.)
"Sir Fred" is Sir Frederick Pottinger, a "monumentally inept" officer of the crown who bungled the whole case -- and eventually managed to accidentally kill himself -- again see "Ben Hall" for background.
To tell this song from the other Ben Hall songs, consider this first stanza:
Come all you sons of liberty and listen to my tale;
A story of bushranging days I will to you unveil.
It's of those gallant heroes, God bless them one and all!
So let us sit and sing: 'God save the King, Dunn, Gilbert, and Ben Hall.'"