In order to identify the gap structure of CeIrIn 5, we measured field-angle-resolved specific heat C(φ) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(φ) exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting H out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the d x2-y2-wave gap. These results strongly indicate that CeIrIn 5 is a d x2-y2-wave superconductor and suggest the universal pairing mechanism in CeMIn 5 (M=Co, Rh, and Ir). © 2012 American Physical Society.