Bill: I am a descendent of the Patrick Cassady you mention as moving from Chesterfield District, SC to Henry Co. AL. Patrick married Elizabeth Nordan and had 3 children, Albert Evan, Harriet, and Sarah Ann. Patrick died, and Elizabeth married Henry Abner Coburn. They moved to Sabine Parish, LA. Sarah Ann married Elias Weldon. Their daughter, Sarah Callie (Weldon) McFarland was my great grandmother.
Elizabeth is buried with Henry Coburn in Middle Creek Cemetery in Sabine Parish. Sarah Ann, Elias, and Callie are buried in Blue Branch Cemetery in Pitkin, Vernon Parish, LA.
Info on "Rootsweb" shows Patrick's parents as Zachariah Cassady (Casity) and Mary Idly. Both born in Ireland and married in Ireland. I've looked a little on the web and found Zachariah in some of the censuses but nothing on Ireland, the marriage, or when they came to the US.
Have you had any better luck? I wonder about Idly. Could be a corruption of Hedly or Headly.