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"

Moby Thesaurus II by Grady Ward, 1.0:

201 Moby Thesaurus words for "irresistible":
   absolute, adducible, admissible, alluring, appealing, appetizing,
   armipotent, attestative, attestive, attractive, authentic,
   authoritative, based on, beguiling, bewitching, blandishing,
   cajoling, captivating, catching, certain, charismatic, charming,
   circumstantial, coactive, coaxing, cogent, come-hither, compelling,
   comprehensive, compulsatory, compulsive, compulsory, conclusive,
   consequential, considerable, constraining, convincing, coquettish,
   cumulative, damning, decisive, deep, delightful, determinative,
   documentary, documented, driving, dynamic, effective, enchanting,
   energetic, engaging, enravishing, enthralling, enticing,
   entrancing, evidential, evidentiary, ex parte, exciting,
   exhaustive, exotic, exquisite, eye-witness, factual, fascinating,
   fated, fateful, fetching, final, firsthand, flirtatious, forceful,
   forcible, founded on, full, glamorous, grand, grave, great,
   grounded on, hearsay, heart-robbing, heavy, high-potency,
   high-powered, high-pressure, high-tension, hypnotic, impenetrable,
   imperative, imperious, implicit, impregnable, in force, in power,
   incontestable, incontrovertible, indefeasible, indicative,
   indisputable, indomitable, ineluctable, inescapable, inevasible,
   inevitable, inexorable, inexpugnable, inflexible, insuperable,
   intense, interesting, intriguing, invincible, inviolable, inviting,
   invulnerable, irrefutable, irrepressible, irrevocable, lovely,
   luxurious, main, material, maximum, mesmeric, mighty,
   mighty in battle, mouth-watering, necessary, nuncupative,
   operative, overpowering, overriding, overwhelming, piquant,
   plenary, potent, powerful, prepossessing, prepotent, pressing,
   presumptive, probative, provocative, provoquant, puissant,
   ravishing, relentless, reliable, resistless, restraining, ruling,
   seducing, seductive, sensuous, serious, significant, siren,
   sirenic, spellbinding, spellful, striking, strong, suggestive,
   sure, sure as death, sure as fate, symptomatic, taking,
   tantalizing, teasing, telling, tempting, thrilling, tickling,
   titillating, titillative, total, unassailable, unavoidable,
   unbearable, unbeatable, unconquerable, uncontrollable,
   undeflectable, ungovernable, unmanageable, unpreventable,
   unstoppable, unsubduable, unsurmountable, unyielding, valid,
   vigorous, vital, voluptuous, weighty, winning, winsome, witching