From the American south, "y'all" is the only term in standard English to compare with the Spanish "vosotros," for example, or the Dutch "jullie" — or the cognate within any other language sensible enough* to have a plural second-person pronoun.

"The plural you is you, the weakness is in ignorance." — Erica, from my guestbook.

Irish English does indeed have a plural "you." If I'm not mistaken, it works like this: When the second person is the subject of the phrase, the Irish will say "youse," [or "yiz," especially in Dublin;] when the object, they will say "ye."   Example: "Are youse comin' over to me gaff? I'll be there before ye."

The major problem with the term "y'all" — our closest match in American — is that it's regional; well-known, but not widely used. Outside of its region, the word itself may draw more attention than does the speaker's intent. It's wildly informal, at best, in some places. Another problem with "y'all" is that, outside of its region, it implies multiplicity — literally, of course, it means "all of you," and not "both of you."

In practice, many English-speaking people say "you guys" to mean "plural you." But the word "guy" is casual, and, besides that — in its singular form — refers to a male individual.

So that's it. We don't really have a "plural you." It's a weakness of the language.

* Conversely, the Dutch and Spanish have to distinguish between formal and informal versions of their "you" cognate, which is kind of silly.