The Collaborative International Dictionary of English v.0.48:

Irresistible \Ir`re*sist"i*ble\, a. [Pref. ir- not + resistible:
   cf. F. irr['e]sistible.]
   That can not be successfully resisted or opposed; superior to
   opposition; resistless; overpowering; as, an irresistible
   attraction.
   [1913 Webster]

         An irresistible law of our nature impels us to seek
         happiness.                               --J. M. Mason.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irresistible
    adj 1: impossible to resist; overpowering; "irresistible (or
           resistless) impulses"; "what happens when an irresistible
           force meets an immovable object?" [syn: irresistible,
           resistless] [ant: resistible]
    2: overpoweringly attractive; "irresistible beauty"