My lady friend is originally from the Mobile area of Alabama, with many years spent in Texas. We discussed "y'all" and "all y'all" a while ago and she informed me that "y'all" is either singular or plural, and "all y'all" is never correct. She considers any use of "all y'all" comic or even mocking.

"Y'all" is often used when the word "you" could be omitted.
Y'all come over after dinner.
It's sometimes added at the end of a sentence.
Come on over after dinner, y'all.