The Grateful Dead, like myself, first heard this song done by The Dixie Cups in 1965. It's called a traditional song, and some say that it has Native American roots.
I seriously doubt this, due to the use of the term "flag boy." A flag boy is one of the folks in the tribes which take part in Mardi Gras each year. The baddest Indian in the tribe is the Spy Boy. He scouts ahead for other tribes, and when he sees them, he tells Flag Boy so that Flag Boy can alert Big Chief. If you don't know what the hell I'm talking about here, you need to visit Nawlins during Mardi Gras some time. It's a hoot.
So I'm guessing that this song was traditional from the Nawlins area and probably the result of some cajun nonsense.
The three girls called the Dixie Cups were sisters Barbara and Rosa Hawkins and their cousin Joan Johnson, from the Calliope housing project in New Orleans. Phil Spector made them stars with "Chapel of Love" in 1964.