William's previous marriage is proving more difficult. Given the ages of William (junior) and Gregg in the 1901 census, there is a potential candidate in the marriage of a William Barton to a Margaret Crane or Crean in Kilskeery Church of Ireland in 1855. Children to this couple include George 1865, Sarah 1870, William 1873, unnamed 1875, and unnamed 1878, though I suspect there may be other children. Anyhow, I wouldn't rule that marriage in or out until you can find out more.
BTW, there is a grave in Magheraculmoney Church of Ireland in the townland of Tullanaglug for a Gregg Barton, died 28 Feb 1964. Maybe William's son... again I wouldn't rule it out just yet.