There is a lot in Wm. Chappell's PMOT on "The Girl I left behind me", but he doesn't give any solid references that one can pin down as to date, and no one has ever been able to verify anything that he said pertaining to an 18th century date. I found the tune direction "The Girl I left behind me" in a songbook of 1799, and that is all I've ever seen that puts the song in the 18th century. I have a copy of the song from an Irish collection of 1805-6, (this appears to be the earliest copy found) and that has no mention of a Brighton camp.

"Garryowen" is from a stage production of 1800, 'Harlequin Amulet', and the tune attributed to a Jackson of Cork (not Walker 'Piper' Jackson). The tune was given earlier as "Auld Bessy" in Aird's 'Airs'.