All 4 Uses

irrational
in
Marley & Me
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We had hoped the chipper-shredder incident in the garage was an isolated aberration, but it turned out to be just the beginning of what would become a lifelong pattern of phobic, irrational behavior.†
p. 72.3

irrational = unreasonable
I even tried my best to indulge her increasingly bizarre and irrational behavior.†
p. 94.9 *
Despite the prescription tranquilizers, which Jenny was feeding him with increasing frequency, more for her sake than for his, Marley's thunder phobia grew more intense and irrational each day.†
p. 137.5
Maybe his fear of thunder had not been so irrational after all.†
p. 193.6

Definitions:

(1) (irrational)
not reasonable
(2) (meaning too rare to warrant focus)
More rarely, irrational numbers in mathematics are numbers that cannot be expressed as a fraction consisting of only an integer in the numerator and denominator. For example, pi is an irrational number while 1/3 is rational.