The suggestion has been made that brumby comes from the Irish Gaelic bromach meaning ‘a young horse or colt’ with a plural form bromaigh pronounced ‘brummy’.
The interesting thing about brumby is that it does not show up until the 1870s.
After that there are newspaper articles about the pest of the wild horses or ‘brumbies’ leading to the shooting of about 7000 of them in western New South Wales and southern Queensland. This of course had to be reported to Sydney readers and the word ‘brumby’ explained because it was assumed that Sydneysiders would not know it.
The most likely theory is that the first brumbies were the horses belonging to a gentleman named Brumby who turned them loose on the Emu Plains when he left for Tasmania. Brumby’s horses became brumbies, but only bushmen and Aboriginal stockman used the term. The word travelled along the inland stock routes and would have remained a lesser-known item of pastoral jargon if it had not been for the fact that they multiplied in numbers to such a degree that they became a newsworthy item. Linguists seem fairly convinced that it is not an Aboriginal word.