With G=GL(n,C), let XΓG be the G-character variety of a given finitely presented group Γ, and let XirrΓG⊂XΓG be the locus of irreducible representation conjugacy classes. We provide a concrete relation, in terms of plethystic functions, between the generating series for E- polynomials of XΓG and the one for XirrΓG, generalizing a formula of Mozgovoy-Reineke [MR]. The proof uses a natural stratification of XΓG coming from affine GIT, the combinatorics of partitions, and the formula of MacDonald-Cheah for symmetric products; we also adapt it to the so-called Cartan brane in the moduli space of Higgs bundles. Combining our methods with arithmetic ones yields explicit expressions for the E-polynomials of the irreducible stratum of GL(n,C)-character varieties of some groups Γ, including surface groups, free groups, and torus knot groups, for low values of n.