Definitely in the public domain! This song has been around, in one form or another, since the time of Shakespeare. Some say it was written by King Henry VIII. The title and lyrics have changed over the centuries, and the tune has been used as a bawdy ballad, a love song, a political ditty, even an accompaniment to executions. My version is intended to be an instrumental solo; it doesn't matter what you call it or where the lyrics come from.
This version has three verses, each different in mood. It's fun to play, and uses lots of slides, hammer-ons, pull-offs, tremolos, and single and double harmonics. Done without fanfare, it tends to stun audiences, which is pretty good for a piece that scarcely lasts three minutes!
I don't have a complete study guide written for Greensleeves, but there is a minimal one at the end of the tab.