The Collaborative International Dictionary of English v.0.48:

Irregular \Ir*reg"u*lar\, a. [Pref. ir- not + regular: cf. F.
   irr['e]gulier.]
   Not regular; not conforming to a law, method, or usage
   recognized as the general rule; not according to common form;
   not conformable to nature, to the rules of moral rectitude,
   or to established principles; not normal; unnatural;
   immethodical; unsymmetrical; erratic; no straight; not
   uniform; as, an irregular line; an irregular figure; an
   irregular verse; an irregular physician; an irregular
   proceeding; irregular motion; irregular conduct, etc. Cf.
   Regular.
   [1913 Webster]

         Mazes intricate,
         Eccentric, intervolved, yet regular
         Then most when most irregular they seem. --Milton.
   [1913 Webster]

         Leading the men of Herefordshire to fight
         Against the irregular and wild Glendower. --Shak.
   [1913 Webster]

         A flowery meadow through which a clear stream murmured
         in many irregular meanders.              --Jones.

   Syn: Immethodical; unsystematic; abnormal; unnatural;
        anomalous; erratic; devious; crooked; eccentric;
        unsettled; uneven; variable; changeable; mutable;
        desultory; disorderly; wild; immoderate; intemperate;
        inordinate; vicious.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irregular \Ir*reg"u*lar\, n.
   One who is not regular; especially, a soldier not in regular
   service.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irregular
    adj 1: contrary to rule or accepted order or general practice;
           "irregular hiring practices" [ant: regular]
    2: not occurring at expected times [syn: irregular,
       unpredictable]
    3: (used of the military) not belonging to or engaged in by
       regular army forces; "irregular troops"; "irregular warfare"
       [ant: regular]
    4: (of solids) not having clear dimensions that can be measured;
       volume must be determined with the principle of liquid
       displacement [ant: regular]
    5: falling below the manufacturer's standard; "irregular jeans"
    6: deviating from normal expectations; somewhat odd, strange, or
       abnormal; "these days large families are atypical"; "atypical
       clinical findings"; "atypical pneumonia"; "highly irregular
       behavior" [syn: atypical, irregular]
    7: lacking continuity or regularity; "an irregular worker";
       "employed on a temporary basis" [syn: irregular,
       temporary]
    8: (of a surface or shape); not level or flat or symmetrical;
       "walking was difficult on the irregular cobblestoned surface"
    9: independent in behavior or thought; "she led a somewhat
       irregular private life"; "maverick politicians" [syn:
       irregular, maverick, unorthodox]
    n 1: a member of an irregular armed force that fights a stronger
         force by sabotage and harassment [syn: guerrilla,
         guerilla, irregular, insurgent]
    2: merchandise that has imperfections; usually sold at a reduced
       price without the brand name [syn: irregular, second]

Moby Thesaurus II by Grady Ward, 1.0:

410 Moby Thesaurus words for "irregular":
   Bohemian, Charley, Mickey Mouse, VC, Vietcong, aberrant, abnormal,
   actionable, adrift, affable, afloat, against the law, aimless,
   alternating, amorphous, anamorphous, anarchic, anarchistic,
   anomalistic, anomalous, anomic, askew, asymmetric, asymmetrical,
   atypical, base, bent, bizarre, black-market, bootleg, bowed,
   broken, broken off, bumpy, bushfighter, bushwhacker, capricious,
   careening, casual, catchy, changeable, changeful, changing,
   chargeable, cheap, cheesy, chopped-off, choppy, coarse,
   coarse-grained, cockeyed, common, contorted, contraband,
   contrary to law, cordial, corrugated, craggy, crank, crankish,
   cranky, crazy, criminal, crooked, cross-grained, crotchety,
   crumpled, crunched, decousu, degage, designless, desultory,
   deviable, deviant, deviating, deviative, deviatory, different,
   disarticulated, disconnected, discontinued, discontinuous,
   discrete, disjunct, disjunctive, disordered, disorderly, disparate,
   dispersed, disproportional, disproportionate, distorted,
   divaricate, divergent, diversified, diversiform, dizzy, dotty,
   easy, easygoing, eccentric, episodic, erose, erratic, exceptional,
   extraordinary, familiar, fast and loose, felonious, fey, fickle,
   fitful, flaky, flawed, flickering, flighty, flitting, fluctuating,
   folksy, formless, fourth-class, freakish, freaky, free and easy,
   frivolous, funny, giddy, gracious, grainy, granulated, gratuitous,
   guerrilla, guttering, halting, haphazard, haymish, herky-jerky,
   heteroclite, heteromorphic, hit-or-miss, homely, homespun, homey,
   horripilant, idiocratic, idiosyncratic, ill-matched, ill-sorted,
   illegal, illegitimate, illicit, immethodical, impermissible,
   impetuous, impulsive, inadequate, inchoate, incoherent,
   inconsistent, inconstant, indecisive, indiscriminate, inequal,
   inferior, infirm, informal, insufficient, intermittent,
   intermitting, interrupted, irresolute, irresponsible, jagged,
   jerky, jolty, justiciable, kinky, kooky, labyrinthine, lawless,
   linsey-woolsey, loose, lopsided, low-class, low-grade, low-quality,
   low-test, lumpy, lurching, maggoty, maquis, maquisard, mazy, mean,
   meaningless, mercurial, mismatched, misshapen, moody, motley,
   mutable, natural, nonconforming, nonconformist, nonconstitutional,
   noncontinuous, nonlegal, nonlicit, nonlinear, nonsequential,
   nonserial, nonstandard, nonsymmetric, nonsymmetrical,
   nonsystematic, nonuniform, nutty, occasional, odd, oddball,
   off-balance, off-key, offbeat, offhand, offhanded, one-sided,
   orderless, out of proportion, outlaw, outlawed, overbalanced,
   paltry, parenthetic, partisan, patchy, patriot, peculiar, pimply,
   pitted, plain, planless, pluralistic, pocky, poor, potholed,
   promiscuous, punishable, punk, purposeless, queer, quirky, ragged,
   rambling, random, rank, relaxed, resistance, resistance fighter,
   restless, ripply, rough, rough-grained, rough-hewn, roving,
   ruffled, rutted, rutty, scatterbrained, schlock, schmatte, scrappy,
   screwball, screwy, second, second-best, second-class, seedy,
   senseless, shabby, shagged, shaggy, shapeless, shifting, shifty,
   shuffling, simple, singular, skew, skewed, snatchy, sociable,
   spasmatic, spasmic, spasmodic, spastic, spineless, sporadic,
   spotty, sprung, staggering, straggling, straggly, strange, stray,
   straying, subnormal, suspended, systemless, tacky, textured, third,
   third-class, third-rate, tinny, tortuous, triable, twisted,
   unaccountable, unaffected, unallowed, unarranged, unassuming,
   unauthorized, unbalanced, unceremonious, uncertain, unclassified,
   uncommon, unconnected, unconsidered, unconstitutional,
   unconstrained, uncontrolled, unconventional, undependable,
   under-the-counter, under-the-table, underground,
   underground fighter, undirected, undisciplined, unequable, unequal,
   uneven, unfixed, ungraded, unique, unjoined, unkempt, unlawful,
   unlevel, unmethodical, unmetrical, unnatural, unofficial,
   unordered, unorganized, unorthodox, unpolished, unpredictable,
   unrefined, unregular, unreliable, unrestrained, unrhythmical,
   unsettled, unsmooth, unsorted, unstable, unstable as water,
   unstaid, unstatutory, unsteadfast, unsteady, unstudied,
   unsuccessive, unsymmetric, unsymmetrical, unsystematic, ununiform,
   unusual, unwarrantable, unwarranted, vacillating, vagrant, vague,
   variable, variegated, variform, various, varying, veering,
   vicissitudinary, vicissitudinous, volatile, wacky, wandering,
   wanton, warped, wavering, wavery, wavy, wayward, weird, whimsical,
   wimpled, wishy-washy, wobbling, wobbly, wrongful



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IRREGULAR. That which is done contrary to the common rules of law; as,
irregular process, which is that issued contrary to law and the common
practice of the court. Vide Regular and. Irregular Process.