A rationale is a reason for something, usually a good reason, wheras an irrationale is a bad or irrational reason for something.
For example, a friend of mine was just telling me why I should use Ruby on Rails instead of Google Web Toolkit. She said it's because VCs won't like you to use technology developed by a big company like Google. Well that won't fly because she also said we should use GOOG Maps. Not making sense at all, quite an irrationale.
by Issac Trotts June 17, 2007