The ion irradiation-induced magnetic transition of L1(0)-MnGa alloy films was studied in detail by measuring X-ray magnetic circular dichroism (XMCD) and the temperature dependence of their hysteresis loops. The XMCD of (001)-oriented MnGa films exhibited different spectral shapes depending on whether magnetization was parallel or perpendicular to the film plane; that is considered to be the origin of the large perpendicular magnetic anisotropy of the L1(0)-MnGa. The anisotropy of XMCD spectra was not influenced by the ion irradiation to the MnGa, while the intensity of the XMCD signal decreased with increasing ion dose. From the measurement of hysteresis loops at low temperature, the significant increase of coercivity H-c in the ion-irradiated MnGa was found by lowering the temperature, while only a slight increase of H-c was observed in the as-prepared MnGa. These results suggest that the ion-irradiated MnGa film has a composite structure of L1(0)-ordered MnGa nanocrystals separated by an A1-disordered MnGa matrix, and that increasing the ion dose results in an increase in the volume ratio of A1-MnGa to L1(0)-MnGa in the film.