Well the ancient Britons stained their skins with woad and had the images of animals etched on to their bodies, so maybe it originated there. If you want to know anything about the tattooing, visit www.tattoo.co.uk which has the history, too.
My thoughts are (since posting the question) that it was worn by sailors as a sign of returning home (the swallow being migatory and returning to Britain each year).
when you get swallow tattoos, they most commenly come as a pair. 1 with more red, the other with more blue. this represents good and evil and choices you make.