The poet is Edna St Vincent Millay, and I found the website that administers her copyrights: http://www.millaysociety.org/millayrights.htm

Thanks for the replies, although now I'm dreading the possibility of them asking "ridiculous amounts of money" for the permission. But now I'm all set to go if and when I ever decide to record the song, if and when I ever learn to play it well enough and talk Frank into singing it!

While on the subject, one of my favorite poems as lyrics is Rani Arbo's "Crossing the Bar" with words by Tennyson.