Ionization energy is defined as the energy required to remove an electron from an atom. Therefore, a positive ionization energy implies that you are looking at an oxidation:
M + hv --> (M+) + e-
a negative ionization energy implies you are looking at the reverse process, namely a reduction:
(M+) + e- --> M + hv
notice that the placement of the energy term hv. It is in the reactant side when a process is endothermic (you gain energy) and on the product side when it is exothermic (you lose energy).