The Collaborative International Dictionary of English v.0.48:

Irregularly \Ir*reg"u*lar*ly\, adv.
   In an irregular manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irregularly
    adv 1: in an irregular manner; "the patient is breathing
           irregularly" [ant: regularly]
    2: having an irregular form; "irregularly shaped solids" [ant:
       regularly]
    3: in an irregular manner; "her letters arrived irregularly"
       [syn: irregularly, on an irregular basis] [ant: on a
       regular basis, regularly]
    4: in an irregular manner; "the stomach mucosa was irregularly
       blackened"

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "irregularly":
   at haphazard, at intervals, at irregular intervals, at random,
   brokenly, by catches, by chance, by fits, by jerks, by snatches,
   capriciously, carelessly, chaotically, choppily, desultorily,
   disconnectedly, discontinuously, eccentrically, erratically,
   every which way, fitfully, haltingly, haphazardly, helter-skelter,
   higgledy-piggledy, hit or miss, impulsively, in snatches, in spots,
   in the rough, inconsistently, inconstantly, indiscriminately,
   infrequently, intermittently, jaggedly, jerkily, lurchingly,
   nonuniformly, off and on, on, patchily, promiscuously, raggedly,
   randomly, rough, roughly, sloppily, spasmodically, sporadically,
   spottily, uncertainly, uncommonly, unequally, unevenly,
   unmethodically, unpredictably, unrhythmically, unsteadily,
   unsystematically, ununiformly, variably, whimsically, wobblingly