Hydrogen (H) can drastically change the physical properties of solids by the doping of host materials with minimum perturbation to the lattice because of its small size, quantum nature, and a variety of charged states from -1 (hydride, H-) to +1 (proton, H+). While the H-doping amount is limited under equilibrium conditions, H2+ ion irradiation at low temperature is a promising method for introducing a large amount of hydrogen into any material. Although the application of this method offers the potential for exploring unforeseen fascinating properties, the effects of nonequilibrium H doping at very low temperature below 10 K are largely underexplored and are not well understood. In this article, we report heavy H (D) doping into ZnO films by H2+ (D2+) irradiation at 7 K, which resulted in metallic conductivity and an isotope effect on the conductivity at 7 K. The H/D isotope effect is attributable to metastable H (D) trapping sites generated by the effect of irradiation. The isotope effect is decreased at low acceleration voltage. Furthermore, the subsequent thermal excursion induces a large irreversible decrease in resistivity, indicating the migration of H (D) from metastable trapping sites upon heating. This work provides a new strategy to control the physical properties of materials and to investigate the H (D) migration occurring with increasing temperature after excess H doping at very low temperature.