I have just posted a new song history on my website:
The Clancy Brothers used to sing this song, some versions from the early 60s are available at YouTube (1962 & 1963, both from TV shows) but the first one to record it was Burl Ives in 1949 on his LP Wayfaring Stranger (Stinson SLP-1).
Brennan was an outlaw in Ireland in the early years of the 19th century. It’s often claimed that he was executed in 1804 but it’s more likely that this happened in 1812. Otherwise not much is known about him but I was surprised to find some magazine articles about a highwayman named Brennan from 1809 and 1812. Here is one from the The Edinburgh Annual Register for 1809 (Vol. 2/2, 1811, p. 44):
Bob Dylan was a great admirer of the Clancy Brothers. His “Ramblin’ Gamblin Willie” (1962) is based on their version of “Brennan On The Moor”. Here is a live performance of Dylan’s song by the Clancys themselves with Robby O’Connell in 1996 as well as one by Liam Clancy in 2005 at a Dylan Tribute show.