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.