The Collaborative International Dictionary of English v.0.48:

Irresponsible \Ir`re*spon"si*ble\, a. [Pref. ir- not +
   responsible: cf. F. irresponsable.]
   1. Not responsible; not liable or able to answer for
      consequences; innocent.
      [1913 Webster]

   2. Not to be trusted; unreliable; lacking a proper sense of
      responsibility.
      [1913 Webster]

WordNet (r) 3.0 (2006):

irresponsible
    adj 1: showing lack of care for consequences; "behaved like an
           irresponsible idiot"; "hasty and irresponsible action"
           [ant: responsible]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "irresponsible":
   adrift, afloat, alternating, amorphous, capricious, carefree,
   careless, changeable, changeful, chartered, desultory, deviable,
   devil-may-care, disobedient, dizzy, eccentric, erratic, excepted,
   excused, exempt, exempted, fast and loose, favored, feckless,
   fickle, fitful, flickering, flighty, flitting, fluctuating,
   fly-by-night, freakish, giddy, headstrong, heady, immune,
   impetuous, impulsive, incautious, inconsistent, inconstant,
   indecisive, ineffectual, infirm, insubordinate, irregular,
   irresolute, lawless, let off, licensed, licentious, mazy,
   mercurial, moody, mutinous, permitted, privileged, rambling,
   rampant, rash, reckless, reinless, released, restless, roving,
   scatterbrained, self-willed, shapeless, shifting, shifty,
   shuffling, spared, spasmodic, spineless, trustless, unaccountable,
   unanswerable, unbridled, uncertain, unchecked, uncontrolled,
   uncurbed, undependable, undisciplined, unfaithworthy, unfixed,
   ungoverned, unliable, unpredictable, unreined, unreliable,
   unrestrained, unruly, unsettled, unstable, unstable as water,
   unstaid, unsteadfast, unsteady, unsubject, unsure, untrustworthy,
   untrusty, vacillating, vagrant, variable, vicissitudinary,
   vicissitudinous, volatile, wandering, wanton, wavering, wavery,
   wavy, wayward, weak, whimsical, wild, wildcat, willful,
   wishy-washy