The Collaborative International Dictionary of English v.0.48:

Irresolvable \Ir`re*solv"a*ble\, a. [Pref. ir- not + resolvable.
   Cf. Irresoluble.]
   Incapable of being resolved; not separable into component
   parts.
   [1913 Webster]

   Irresolvable nebul[ae] (Astron.), nebul[ae] of a cloudlike
      appearance, which have not yet been resolved by the
      telescope into stars, and are often distant galaxies. See
      galaxy[2]. --Sir W. Herschel.
      [1913 Webster +PJC]