Even though the only B I ever got in graduate school was in a linguistics course, articles like this still warm the cockles of my old English major heart. And there are a lot of Scoch-Irish around here, so the theory below seems to ring true. From Salon.
The secret history of “Y’all”: The murky origins of a legendary Southern slang word
In academic circles, many subscribe to Michael B. Montgomery’s suggestion that “y’all” descends from the Scots-Irish “ye aw” and not directly from “you all.” He cites a 1737 letter by a Scots-Irish immigrant in New York as an example: “Now I beg of ye aw to come our [over] here.”
Montgomery’s argument relies on two observations about “y’all’s” unique place among English contractions. First, contractions in English place stress the first word and contract the second, such as in the case of “they’re,” where “they” is stressed while “are” has been shortened. But “y’all” does not conform to this pattern. Instead, it stresses the second word, “all,” and contracts the first, “you.” Secondly, there are no other contractions that involve “all” in English, whereas we have lots of contractions involving “will,” “not,” and “are.” These irregularities suggest a more complex origin, such as a cognate word, like “ye aw.”