It is indeed a setting for Yeats’ poem. I understand that Tippett initially intended the piece to be sung by Jessye Norman but she declined very close to the premiere. Faye Robinson stood in and definitely made the piece her own! I can still remember her performance at the Royal Albert Hall as one of the most outstanding classical concerts I have ever attended. Read all about the premiere in Chicago at http://www.nytimes.com/1991/04/17/arts/review-music-tippett-s-byzantium-by-solti-and-chicago.html .
If you like something with a bit of melody, you might enjoy George Lloyd’s Piano Concerto No.4 with Kathryn Scott.