Angelus wasn't Angelus; he was Liam. Spike was William. "Darla" wasn't a name when Darla was turned. And we have to assume that a pious Catholic family wouldn't have named a daughter Drusilla. What, then, was Drusilla's name?
Answer: We don't know, but I strongly suspect it was Agnes. Three iterations of the song about the lamb in the blackberry patch is a bit too much for an arbitrary detail.