Fake sincerity or fake sarcasm, and why?

Fake sincerity comes in kinds:

politeness,

acting on stage,

deception.

Irony depends on context to express a meaning other than the words spoken, typically for humorous or emphatic effect. The listener or reader must know the context in order to understand the meaning.

If irony is used to mock a thing, and idea, a person or something said, it is often used with a scornful or nasty tone of voice. This can often be lost in the written word, unless the context is clear.

The same words spoken in a friendly tone could sound like "fake sarcasm," a playful nip that stops short of drawing blood.

(Pups need to be bitten to learn what hurts and what doesn't.)

False sincerity or fake sarcasm, both depend on the intent of the speaker.

Neither.  And why should not even enter into the picture. 

What's fake sarcasm? Is it pretending to be sarcastic when you're actually being truthful?

But anyway, neither. I don't believe in being fake anything :P

So "xoSapphira" is your real name, right?


(Lol.)
~

Yes, and my avatar is actually how I look like. :P

"pretending to be sarcastic when you're actually being truthful"

I think you hit the nail on the head.

I think a great many people do that and get away with it with the victim having no idea of the truth.