Brave Danny Flint
"Brave Danny Flint" is a sad and pretty song about Danny Flint, a girl who disguised herself as a boy to join the Night's Watch but ended up raped and murdered by the black brothers. In some versions of the song her ghost still walks the Nightfort. 
A Dance with Dragons
When Tormund Giantsbane brings the wildling hostages to the Wall, Jon Snow spots two girls disguised as boys. Jon requests for them to be replaced, but Tormund argues that a hostage is a hostage. Jon asks if Mance Rayder ever sang of Danny Flint. Tormund does not recall Mance having done so.