The Collaborative International Dictionary of English v.0.48:

Irremissible \Ir`re*mis"si*ble\, a. [L. irremissibilis: cf. F.
   irr['e]missible. See In- not, and Remissible.]
   Not remissible; unpardonable; as, irremissible crimes.
   --Burke. -- Ir`re*mis"si*ble, n. -- Ir`re*mis"si*bly,
   adv.
   [1913 Webster]