Synonyms for irrational

modified

1. Illogical

   unreasonable, specious, fallacious, untenable; see illogical, wrong 2.

2. Stupid

   senseless, silly, ridiculous, absurd; see stupid 1.

irrational implies mental unsoundness or may be used to stress the utterly illogical nature of that which is directly contrary to reason an irrational belief that everybody was his enemy; unreasonable implies bad judgment, willfulness, prejudice, etc. as responsible for that which is not justified by reason unreasonable demands