  Quasi-two dimensional CeIrIn5 contains two distinct domes with different
heavy fermion superconducting states in its phase diagram. Here we pinned down
the superconducting gap structure of CeIrIn5 in the second dome, located away
from the antiferromagnetic quantum critical point, by the thermal transport
measurements in magnetic fields rotated relative to the crystal axes. Clear
fourfold oscillation was observed when the field is rotated within the
ab-plane, while no oscillation was observed within the bc-plane. In sharp
contrast to previous reports, our results are most consistent with d_{x^2-y^2}
symmetry, implying that two superconducting phases have the same gap symmetry
which appears to be mediated by antiferromagnetic spin fluctuations.
