Watch her fight with Zachary carefully. He stabbed her through the center of her chest (not even through her heart) with a metal music stand, not a wooden stake. She was still very much alive (or, well, still undead) when he left her to chase Ash instead. Easy shelter was also close by, in the open access tunnel to the stage that Zachary and Sophia came up through; she very likely managed to painfully crawl the short distance to safety.
It could be argued that she was the real winner of this film. Her master is finally destroyed, freeing her from his thrall, and she is easily the most powerful, senior vampire of those few that remain after Zacharys rampage. Iris would likely get a nasty surprise the following night when Cassandra returned and announced that she was in charge now, and promptly took over Ashs old suite.