Looking at your render, the clipping is obviously there in both - the hair seems to disappear into the breasts at the same spot. What isn't showing up in the preview is the part of the hair that's protruding from the bottom of her breasts. So, it isn't a question of "not intersecting," it's a question of "why are those bits not being rendered."
One possibility is the fact that the preview window isn't being shown at "full scale" (i.e. its dimensions are smaller than the final render) and, as a result, the way that iRay is going about doing its sampling is somewhat different than in the final render. I don't have an immediate solution, although it's possible that if you looked at the iRay preview in the Perspective window and were zoomed in a bit, the clipping would be more evident.
If you posted this question on the Daz forums, somebody over there might be more knowledgeable.