Let p = x^3 - x^2 - 1.

Taking on faith that F is actually a field (i.e that p is irreducible in Z5[x]), your question amounts to: Is the zero element/polynomial irreducible?

That's because in Z5[x]/(p), p = 0, or more accurately, p+(p) = 0+(p).

This is one of those special case situations where you have to read the definition of irreducible carefully.

I don't have a text nearby, so I'll use wikipedia:

1) "For any field F, the ring of polynomials with coefficients in F is denoted by F[x]. A polynomial p(x) in is calledirreducible over Fif it is non-constant and cannot be represented as the product of two or more non-constant polynomials from F[x]."

2) "In abstract algebra, a non-zero non-unit element in an integral domain is said to be irreducible if it is not a product of two non-units."

In both cases, the special case of a 0 element would be excluded. Thus I read that as 0 should not called irreducible.