From what I read he was already dead when the medicals arrived. As you said Threehorn, it doesn't make any real difference. The articles I read pointed out that, while the poison of sting rays usually isn't lethal to humans (Irwin is the third Australian in history to die from a sting ray sting) the sting itself shouldn't be underestimated. In the article it was compared to a bayonet.
I'm afraid that due to all his experience Irwin probably grew somewhat to careless about the risk, that may come from the fascinating creatures he was dealing with, if they feel threatened.