Background: Refers to Irish Rebellion of 1798 - inspired by French Revolution mainly. Often quoted as the last time Catholics and Protestants fought on the same side against the "British" - but things are never that simple in Ireland! It was only some protestants (presbyterians) and they were careful to fight at opposite ends of the country!
"croppy boy" is said to come from the nasty habit at the time of shaving the scalps of rebels and covering them with pitch.
There is a second song of the same name to a different air.
I think the song was written for the centenary - not sure. It and many others will get a fresh airing next year!