The Collaborative International Dictionary of English v.0.48:

Irresistibility \Ir`re*sist`i*bil"i*ty\, n. [Cf. F.
   irr['e]sistibilit['e].]
   The quality or state of being irresistible, irresistibleness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irresistibility
    n 1: the quality of being overpowering and impossible to resist
         [syn: irresistibility, irresistibleness]