It is always best to hang from a solid structure like a stud. This does away with ever having to worry that the fixing in the plasterboard is strong enough. There is a variety of products to hang a mirror with and it depends on what type of fixing it has on the back.
If the mounting points will not line up with your stud work, then you could place a bridging piece of timber fixed between the studs and fix the mirror to that.
If you want the mirror flush against the plaster wall, you'll be limited to using the screws provided in products like GripIt 15mm Yellow Plasterboard Fixing - 4 Pack. There are no minimum requirements listed, so I would encourage you to test the screws to ensure the heads lock into the mounting plate. If in doubt, please contact the mirror's supplier for further details.