Let X 0 denote a compact, simply-connected smooth 4-manifold with boundary the Poincaré homology 3-sphere Σ(2, 3, 5) and with even negative definite intersection form Q X0 = E 8 . We show that free Z/p actions on Σ(2, 3, 5) do not extend to smooth actions on X 0 with isolated fixed points for any prime p > 7. The approach is to study the equivariant version of the Yang-Mills instanton-one moduli space for 4-manifolds with cylindrical ends. As an application we show that for p > 7 a smooth Z/p action on # 8 S 2 × S 2 with isolated fixed points does not split along a free action on Σ(2, 3, 5). The results hold for p = 7 if the action is homologically trivial.