Yes, it's absolutely irreducible.   The standard reference is R. Steinberg's 1963 paper <a href="http://www.ams.org/mathscinet-getitem?mr=0155937">*here*</a> (which is freely available online).    His 1967-68 Yale lectures, now published by AMS in a LaTeX version, may be a good alternative source.   

It should be emphasized, however, that our knowledge of these irreducibles is indirect and incomplete.    This is somewhat parallel to the infinite dimensional theory in characteristic 0, but far more complicated to resolve.