Sarcasm is never friendly; it's necessarily at the expense of someone else. The simplest example is where someone makes a bold claim and someone else says 'sure, buddy...' to express contemptuous disbelief via the weakest possible form of assent. The claimant here wants to be believed, or at least agreed with.

Irony is imho much more complex and variegated, but a simple example would be any sort of self-deprecating humor, where someone is making fun of the mismatch between their aspirations and their capacity to achieve them. Irony isn't necessarily mean, whereas sarcasm is always a little bit mean even if it's mild.

To be clear, I don't disagree with you, and to say otherwise wasn't my point. I do think people get confused to the point where they for some reason start labeling all sarcasm as type A or B, and the entire value of the term gets lost.

The way I see it, the non-friendly type shares a lot in common with the concept of shibboleth. Which is to say, you can absolutely make sarcastic insults to the detriment of someone else for your own, or a friend's enjoyment, by relying on shared exclusive knowledge. (In essence, holding that shared context above the other person in contempt) However, you can also just be abrasive for your own enjoyment, and that's something entirely differently. (Sadism, for example, is not inherently sarcastic) People frequently confuse the two, but without ironic context - a knowledge of false belief - it is not ironic, and therefore not sarcasm.