It isn't the religious believer that is irrational, it is believing in things with a dearth of evidence. A person can be rational most of the time and still commit irrational acts. People aren't robots with binary (1=rational, 0=irrational) programming.
People, ALL people, are sometimes rational and sometimes irrational. Some people are irrational in many areas and rational only in a few. Others are rational most of the time and only succumb to irrational thoughts and behaviors on the rare occasion.
Why then, are religious believers often classified as irrational? Because they are quite often of the former group. In addition to believing the universe began with a singular intelligent act, they also often believe dozens of other things that are irrational: there are invisible beings all over the place, there is an invisible being alive right now who was a visible man a couple thousand years ago, that same invisible being has the ability to save your eternal soul, everyone has an eternal soul that lives after you die, sometimes these souls get to go to a place called heaven where you can be with the invisible being that started the universe, sometimes these souls stay on earth and play grab-ass with Jay, Grant, Amy, and Kris on the SciFi channel, sometimes these souls go to a bad place where they suffer, around 4000 years ago the entire earth was covered with water for more than a year and the only way humanity and animals survived was because a guy built a big wooden box for every type of animal to fit into, there was once a battle of bronze-age tribesmen in which one tribal leader asked for daylight savings and the invisible being that began the universe held the sun in place for him, a donkey once spoke to its master, around 2000 years ago a virgin had a baby, need I go on?
Why do non-believers sometimes consider believers irrational? In light of the evidence, it seems it would take a great deal of restraint NOT to.