While it is not an exclusive-to-Dallas word, it is hard to go a day in the city without hearing it. It's not limited to a particular age group, gender, social class, or profession.
Jacob: “Morning y’all. Hope y’all working hard today – don’t forget we got a big inspection later on this week.”
Liam: “Morning boss – yup, all going to plan.”
Jacob: “Great news Jacob! Martha, y’all set?”
Martha: “Sure thing!”
It is used for addressing a group of people, or asking a single person about a collective group. It can be used, for example to inquire about somebody’s family, as in “how y’all doin?”
This compilation clip shows the character, Tami Taylor, from the TV show Friday Night Lights, using the phrase y’all… a lot!