Shannon is a SURNAME and a place name in Ireland. It became a first name in the US first for boys, then girls, IIRC.
Many names evolve from being boys' names to girls' names, but not usually the other way around.
Examples:
Stacy/Stacey
Tracy
Courtney
Kim (well, Kim as a girl's name is short for Kimberly, and is a man's name by itself, but anyone called "Kim" was generally a male before 1950)
Shannon
Dana
More recent examples:
Devin/Devon
Taylor
I wonder when "Barak" will break the top 20 in baby names.
Interesting.....