WordNet (r) 3.0 (2006):

irrelevant
    adj 1: having no bearing on or connection with the subject at
           issue; "an irrelevant comment"; "irrelevant allegations"
           [ant: relevant]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "irrelevant":
   adrift, alien, back-burner, beside the mark, beside the point,
   beside the question, dinky, dispensable, extraneous, extrinsic,
   foreign, gratuitous, ill-adapted, ill-assorted, ill-chosen,
   ill-considered, ill-fitted, ill-matched, ill-seasoned, ill-sorted,
   ill-suited, ill-timed, immaterial, impertinent, improper,
   inadmissible, inapplicable, inapposite, inappreciable,
   inappropriate, inapt, inauspicious, incidental, inconsequent,
   inconsequential, inconsiderable, inconvenient, inept, inessential,
   inexpedient, infelicitous, inferior, inopportune, insignificant,
   intempestive, intrusive, irrelative, late, little, mal a propos,
   maladjusted, malapropos, minor, minute, misjoined, mismatched,
   mismated, misplaced, mistimed, negligible, nihil ad rem,
   nonessential, not at issue, not vital, off base, off the subject,
   out of character, out of joint, out of keeping, out of line,
   out of phase, out of place, out of proportion, out of season,
   out of time, out of tune, out-of-the-way, parenthetical, petit,
   premature, small, technical, too late, too soon, unadapted, unapt,
   unbecoming, unbefitting, uncalled-for, unconnected, unessential,
   unfavorable, unfit, unfitted, unfitting, unfortunate, unhandy,
   unhappy, unimportant, unimpressive, unlucky, unnoteworthy,
   unpropitious, unqualified, unready, unrelated, unripe,
   unseasonable, unseemly, unsuitable, unsuited, untimely, untoward,
   wrong