What's the meaning of "Y yo voy y me lo creo"? I encountered it in a Spanish novel. With 146,000 Google.es hits, it seems to be a set expression. Context helps, but doesn't remove all doubts.
I agree it is a sarcasm, something people say when they are told something unbelieveable, stating they are believing it although they actually don't.
It is of very common use here in Spain, mostly from people belonging to a low social/economic condition.
As a nuance, it has to do with self consideration. Saying that you are stating
I am not that silly to believe it.
It literally means "And I go and believe that."
It's a sarcasm, as insulting oneself for being too credulous.
I think is an idiom, a good translation could be: "I can't believe I was silly enough to believe that" or "I'm such an idiot to believe in that". You use it when you discover you were fooled and you're offended by that, mostly with the people who fooled you and with you for being so credulous.