As an atheist it is clear to me that I do not have an irrational belief in the non-existence of god. No, it's more subtle than that. It is more correctly a lack of belief. And more specifically a lack of belief in any of the so far suggested gods, be it mono or polytheistic, across the span of homo's cultural heritage.
Then some might suggest that because the universe is so ineffable that it is impossible to know if there isn't some sort of god. And here is where the argument becomes laughable because just how is it, exactly, that you're defining 'god'. A 'creator', a 'meddler', a 'thinker', a 'cold and callous automaton', with a body or ethereal, what type of body, are they alive, do they have sex, a subset of those descriptions perhaps reductio ad absurdum. At this point the definitions become so specific or broad as to be ridiculous. Why should I believe in these entities either?
I think science and a rule of thumb is most appropriate here. I believe in it if the proof is incontrovertible (evolution, general relativity, the 2nd law etc). I will accept it as fact if the evidence is strong even if the proof is not forthcoming (Riemann hypothesis). The rest should be dealt with with a healthy dose of scepticism (Elvis lives, the tooth fairy, doorways to other worlds in the cupboard).