28 synonyms for irrational such as barmy, absurd, and arbitrary. Irrational is an adjective, and its antonyms are rational and logical.
Absurd, arbitrary, backasswards, barmy, baseless, crazy, daft, foolish, groundless, idiotic, illogical, implausible, laughable, ludicrous, mad, nonsensical, ridiculous, senseless, silly, stupid, unfounded, unjustifiable, unreasonable, unscientific, unsound, untenable, and wild.