It's always been my understanding that IR doesn't necessarily preceed PCOS; it's a 'chicken and egg' question: which came first?
My understanding of the syndrome is that the hormone disruptions involved in PCOS play a part in creating the IR which creates an environment for further hormone disruptions and a downward spiral. Of course, I'm basing this on the fact that not all women who have PCOS also have IR; only about half do. Based on that, there must be at least one other cause for PCOS other than IR or (what I tend to lean towards), IR is a result of PCOS, not a primary cause of it.