Hmmm I think it was made up by either
Vanessa Redgrave (who has a daughter named Joely) or
Phil Collins, who also has a Joely. They both pronounce it JO-lee and as they are the only people with this name (or at least were at the time, I suppose) I guess JO-lee is correct. It makes sense, because I guess their parents combined
Joe (
JO) and
Lee (
LEE). It never occured to me to say JO-eh-lee.