While I don't have the answer to your question I know someone who might. Professor McCracken, a descendant of Henry Joy is, I believe , a Department Head in a Soth African University, possibly Cape Town. He has written a book about the Irish in South Africa and available through the Ulster Historical Foundation in Belfast (12 College Square East). They also have a web site. I'm sure they could tell you how to contact him. Most likely he has worked out the genealogy of his family.