The Collaborative International Dictionary of English v.0.48:

Irreversible \Ir`re*vers"i*ble\, a.
   1. Incapable of being reversed or turned about or back;
      incapable of being made to run backward; as, an
      irreversible engine; an irreversible process; an
      irreversible reaction.
      [1913 Webster +PJC]

   2. Incapable of being reversed, recalled, repealed, or
      annulled; as, an irreversible sentence or decree.
      [1913 Webster]

            This rejection of the Jews, as it is not universal,
            so neither is it final and irreversible. --Jortin.

   Syn: Irrevocable; irrepealable; unchangeable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

irreversible
    adj 1: incapable of being reversed; "irreversible momentum
           toward revolution" [ant: reversible]