Ferromagnetic compound CeAgGa crystallizes in CeCu2Imma structure with Ag and Ga atoms distributed randomly at 8h position. We report X-ray photoemission (XPS) valence-band spectra for CeAgGa and compare the results with those obtained from the full potential linear augmented plane-wave (FP-LAPW) method. In order to study the influence of the atomic disorder at 8h sites on the calculated density of states (DOS) we analysed different geometries of the unit cell, possible for CeAgGa. We found that neither disorder nor Ce 4f electron correlations, when they are also taken into account, do give satisfactory agreement between the calculated DOSs and the XPS spectra. We show, however, that inclusion of a Coulomb repulsion U on the Ag 4d states yields a good result. We also report that disorder in Ag/Ga layers leads to the localization of the Ag 4d states. Magnetic moments on Ce atoms from our calculations are in good agreement with those obtained experimentally and the ferromagnetic ground state is energetically preferred. The valence of Ce ions close to 3+ has also been confirmed by the Ce 3d XPS spectra.