The Collaborative International Dictionary of English v.0.48:

Irrepealable \Ir`re*peal"a*ble\, a.
   Not repealable; not capable of being repealed or revoked, as
   a law. -- Ir`re*peal"a*ble*ness, n. -- Ir`re*peal"a*bly,
   adv.
   [1913 Webster]