Synonyms for Irrational:

adj.

absurd (adjective)
implausible, crazy, ludicrous, ridiculous, foolish, outlandish, preposterous, illogical, farcical, impossible, idiotic, bizarre, invalid, anachronistic, paradoxical, senseless, nonsensical, silly, inane, incongruous, contradictory, fallacious, inconsistent, absurd, self-contradictory, meaningless, eccentric.
deranged (adjective)
nutty.
emotional (adjective)
hysterical.
extreme (adjective)
extreme.
foolish (adjective)
kooky.
illogical, senseless (adjective)
foolish, reasonless, kooky, ridiculous, demented, fallacious, delirious, nutty, silly, absurd, crazy, nonsensical, incoherent, preposterous, insane, mad, invalid, specious, wrong, unreasoning.
insane (adjective)
crazy, lunatic, deranged, hysterical, delirious, schizoid, blithering, nutty, frenzied, babbling, insane, batty, kooky, psycho, Crack-brained, berserk, psychotic, mad, brain-sick, unbalanced, maniacal, neurotic, feverish, demented.
lunatic (adjective)
schizoid.
thoughtless (adjective)
unreasoning.
uncontrolled (adjective)
blind.
unintelligible (adjective)
incoherent.

n.

extreme, childish, perverse, irresponsible. Unreasoned, out of bounds, reason. illogical (noun)
untenable, wrong, specious.
irrational (noun)
jaundiced, unreasoning, blind, nonrational, superstitious, yellow, reasonless, incoherent.

Other synonyms:

perverse, wrong, Unreasoned, childish, irresponsible. extreme. Other relevant words:
jaundiced, extreme, specious, unreasoning, untenable, wrong, reasonless, irresponsible, yellow, blind, childish, nonrational, incoherent, superstitious, Unreasoned.

Usage examples for irrational

  1. So we end in an irrational reverence of that which is worthy of a real and rational reverence. – The Right and Wrong Uses of the Bible by R. Heber Newton
  2. By commending irrational retrogression from active use of the understanding back to dreamy contemplation? – Rousseau Volumes I. and II. by John Morley
  3. 2. Scientifically: Cleanliness, as practised, is conventional and irrational – A Poor Man's House by Stephen Sydney Reynolds