So I had heard of Tam Lin, researched the ballad(s) online (and listened to a few musical reels by the same name), and I had been involved off and on in SCA and Markland events when I was younger, so I had an inkling about medieval life.
I took a few courses in history in college and was an avid attendee of a Renaissance Faire or two in my wild teenage years. I had a fine appreciation of “fairy tales” and certainly read my share of fantasy works.
It wasn’t a big surprise then that I felt the Romance and Fantasy genres could beautifully blend. And certain Mercedes Lackey, Barbara Hambly, RA MacAvoy, Anne McCaffrey, and Robin McKinley books helped cement my opinion.
I (much later) attended a large Celtic-inspired event and spoke to a few families/clans there and gathered assorted information to further round out my research.
So the seed was planted long, long ago and only sprouted when I stumbled on the Tam Lin ballads, since then I've watered it with research and curiosity.
Go and explore!