What is the possessive of ally?

Answer 1

There are two possibilities here: if you mean something belonging to several allies, you would call it the allies'. If you mean something belonging to a single ally, it would be your ally's. Allies without an apostrophe is simply a plural. Allie's written like this, is incorrect, and reads most closely as something belonging to a girl or woman named Allie.

Answer 2

Ally’s