And if you need a plural, it is "all y'all".
HAH! Don't even try to tell a Texas girl about y'all. I had a Yankee neighbor who accused me of using y'all wrong when I would say to her, "Y'all come over to dinner sometime."
She thought I was referring to her (singular) as y'all. Of course, I meant her, her husband and their two children... in other words: all y'all.