Well, I have heard in real "half nine" used by an Irish colleague. And looking up on the internet, there's similar confusion[1], and someone clarifies "half nine = 9.30"
And thank you for the "vol" / "heel" distinction in this context. I feel that definitely is going to stick! :-)
What "half negen" means is easier to remember when you contrast "half" to "vol"/"heel" -> half nine is not full nine yet.
To be clear, nobody uses "vol negen" or "heel negen", it's just "negen (uur)".