The author was John Keegan Casey (Leo), but how old he was I have no idea. 19th [early 20th?] century; the song is about the 1798 rebellion, though, like many others, e.g. "The Croppy Boy", "Boolavogue" etc. This one is often sung to "The Wearing of the Green", but another more dramatic tune is in Colm O Lochlainn's More Irish Street Ballads (1965), no. 67.