I was hanging around a paleo forum I frequent, and there were of course the standard posts from people worried about cholesterol in their eggs or posting meat-heavy recipes, but one in particular was from a very recent ex-vegan who wound up leaving because she is skeptical of the paleo consensus that dietary cholesterol is not harmful.
Somebody left the response "Vegans are irrational."
I am an ex-vegan. I wound up leaving after three years due to progressing health problems (weight gain, high blood pressure, high cholesterol, pre-diabetes, food addiction issues) that were being brushed off by my vegan friends and dismissed as exaggeration by non-friends. I mention that because I am no friend of veganism. I find the idea that I must go vegan, based on my personal health history, insulting. When a friend asks me if I would personally suggest trying veganism, I say "no," especially if they are asking me if I feel it would be good for a specific end (usually weight loss).
But are vegans irrational? Vegan culture is a lot of negative things, but so is paleo culture... and when we make statements about the supposed irrationality of certain groups, we are making the assumption that everybody is--or should be--interested in the same ends.
The primary reason people go vegan is for animal rights purposes. These are people who believe that humans should not be exploiting animals for our own gain, they draw comparisons between animals and oppressed human groups. Although I disagree, this isn't an irrational belief. And when you believe in this not-quite-irrational belief, going vegan itself isn't irrational.
I need to make a big distinction here... there's a difference between being irrational and being deceitful or rude. I feel it's perfectly acceptable to call out individual vegan claims to health... and having been one of those people who wound up gaining fifty pounds on veganism and acquiring health problems I didn't have before, I do bring this up when people ask about veganism for health purposes. Using sketchy health claims to justify moral beliefs is deceitful, and being a total jerk about non-vegans is, well, rude. It's not just an ethical consideration, though; people are more likely to be long-term vegans for animal rights purposes than for health purposes.
That's not something unique to veganism, though. I've left plenty of paleo communities because people were convinced that paleo diets are automatically healthy and easy for everybody, and there's a fair share of woo from people who discourage vaccines (which is risky), encourage consumption of raw meat, including pork and chicken (even more risky), believe that you can literally eat as much as you want and lose weight (not true), and tell people their doctors are wrong without knowing that person's medical history (because it's not like doctors go to medical school or something). So if we're going to call people irrational and cultlike, we must concede that this behavior is a part of the territory of being in any restrictive diet rather than a uniquely vegan trait.
Regardless of your opinion of veganism as a diet, a philosophy, or a lifestyle, calling it "irrational" is ignorant.