  In order to identify the gap structure of CeIrIn5, we measured
field-angle-resolved specific heat C(phi) by conically rotating the magnetic
field H around the c axis at low temperatures down to 80 mK. We revealed that
C(phi) 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 dx2-y2-wave gap. These
results strongly indicate that CeIrIn5 is a dx2-y2-wave superconductor and
suggest the universal pairing mechanism in CeMIn5 (M = Co, Rh, and Ir).
