Wouldn't say optimism is irrational. There are good things happening in the world in spite of all the bad things in the world.

Pessimism that leads to a self fulfilling prophecy is irrational, but you still need a win. A win is fuel.

Choosing a belief that is more desirable than the most likely case, is by definition irrational, and can be called optimistic.

Choosing a belief that is less desirable than the most likely, is equally irrational, clearly pessimistic, and often self-fulfilling.

So the ideal belief system is irrational (optimistic) but only to a chosen and realistic extent.

Somewhere between Pollyanna and Eeyore, but more P than E. And as irrational psychologies go, moderate-P is by far the more successful of the two.