The dimorphism of the intermediate valence ternary cerium silicide CeIr 2 Si 2 in the ThCr 2 Si 2 (α) and CaBe 2 Ge 2 (β) modifications is addressed in the framework of the density functional theory. The geometry optimization is in good agreement with the experiment and the subsequent establishment of the energy–volume equation of state (EOS) indicates a stabilization of the β-type relative to the α-type concomitant with the trend of the cerium valence, changing to tetravalent in β-CeIr 2 Si 2 . This is equally shown from the site projected DOS and from the large increase of the electronic contribution to the specific heat. The chemical bonding indicates the strongest bonding interactions within the Ir–Si substructure in both varieties. Stabilization of β-CeIr 2 Si 2 with almost tetravalent cerium is in good agreement with Th IV Ir 2 Si 2 which exclusively crystallizes in the CaBe 2 Ge 2 type. The EOS behavior of different RIr 2 Si 2 (R=Th, Ce, La) is comparatively discussed.