I think when you say optimism is irrational, you are begging the question etc. There is nothing inherently rational or irrational about optimism -- irrationality comes if your map doesn't show you the territory; that is, if you think things that aren't true. Sometimes situations call for optimism, sometimes pessimism. Sometimes you don't have enough data for either. And yeah, you've got stuff like the planning fallacy, which states that humans tend to be too optimistic even though actual events may not suggest that reaction. But equally can you be too pessimistic. Some people wallow in pessimism, though they have little to be unhappy or skeptical about. That is irrational. Being optimistic to the degree that you have cause to be is not. As an aside, I wouldn't be surprised if being optimistic (not to the point of blatant irrationality, but just as a general rule) has a small positive impact on events in your life. Call it the cheerfulness effect.
I have pretty much already concluded that optimism is bunk and I'm looking for disproofs, spot on. There is statistical evidence (that I can't be bothered to find now) that indicates that an optimistic outlook positively affects outcomes of all kinds. If I were as logical as I sell myself as being, I would be optimistic purely for statistical benefit. It just all feels wrong, given the totality of my understanding of the world.I think when you say optimism is irrational, you are begging the question