Well, let's see. The singer introduced it as "not a Christmas song" (it was a seasonal song circle) so that should narrow it down!
She said it is often sung in the festive season. It was most definitely not a romantic love song nor a religious song, but rather the reign of love refers to love among humanity ruling the Earth.
Not trad, and pretty sure it was a female songwriter.
All not terribly helpful, I suspect. :-(