Pressure and temperature dependences of the electronic structure of the heavy-fermion superconductor CeIrSi3 have been investigated using partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy at the Ce L-3 edge. Ce is in a weakly mixed valence state at ambient pressure, mostly f(1) with a small contribution from the f(0) component. Pressure-induced increase of the Ce valence becomes apparent above 4 GPa, concomitantly with the disappearance of the superconductivity. No temperature dependence of the Ce valence is observed within the measured temperature range down to 24 K.