|Sunrise on the Dead Sea, from our hotel window|
Atheism as philosophical naturalism/materialism (PN; AKA physicalism) must deny the objectivity of value, morality, free will, consciousness, and the like. See atheist Thomas Nagel, who understands that PN cannot carry such freight - Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature Is Almost Certainly False. For some real, heavyweight philosophical fun see J.P. Moreland's Consciousness and the Existence of God: A Theistic Argument.
Atheistic-PN denial of free will seems akin to disbelief in one's own existence. Note that an evolutionary explanation of morality like Sam Harris's commits the naturalistic fallacy; viz., the misguided idea that one can derive 'ought' from 'is.' (See philosopher Colin McGinn's observation here, to which Harris fails to adequately respond; see Kwame Anthony Appiah's review here. Here Oxford philosopher of ethics Simon Rippon argues that Harris commits the naturalistic fallacy.)
Is atheism irrational? For such reasons: yes.
See the recent nytimes interview with analytic philosopher Alvin Plantinga and University of Notre Dame philosopher gary Gutting - "Is Atheism Irrational?" Here is Plantinga explaining one problem the noetic framework of atheism faces.
Gutting: Especially among today’s atheists, materialism seems to be a primary motive. They think there’s nothing beyond the material entities open to scientific inquiry, so there there’s no place for immaterial beings such as God.
Plantinga: Well, if there are only material entities, then atheism certainly follows. But there is a really serious problem for materialism: It can’t be sensibly believed, at least if, like most materialists, you also believe that humans are the product of evolution. [Note: Plantinga is going to point out an internal inconsistency of PN.]...
Gutting: So your claim is that if materialism is true, evolution doesn’t lead to most of our beliefs being true.
Plantinga: Right. In fact, given materialism and evolution, it follows that our belief-producing faculties are not reliable.
Here’s why. If a belief is as likely to be false as to be true, we’d have to say the probability that any particular belief is true is about 50 percent. Now suppose we had a total of 100 independent beliefs (of course, we have many more). Remember that the probability that all of a group of beliefs are true is the multiplication of all their individual probabilities. Even if we set a fairly low bar for reliability — say, that at least two-thirds (67 percent) of our beliefs are true — our overall reliability, given materialism and evolution, is exceedingly low: something like .0004. So if you accept both materialism and evolution, you have good reason to believe that your belief-producing faculties are not reliable.
But to believe that is to fall into a total skepticism, which leaves you with no reason to accept any of your beliefs (including your beliefs in materialism and evolution!). The only sensible course is to give up the claim leading to this conclusion: that both materialism and evolution are true. Maybe you can hold one or the other, but not both.
So if you’re an atheist simply because you accept materialism, maintaining your atheism means you have to give up your belief that evolution is true. Another way to put it: The belief that both materialism and evolution are true is self-refuting. It shoots itself in the foot. Therefore it can’t rationally be held.
For more read the entire interview.
For even more see Plantinga's recent book Where the Conflict Really Lies: Science, Religion, and Naturalism.