  The thermal conductivity $\kappa$ of the heavy-fermion superconductor
CeIrIn$_5$ was measured as a function of temperature down to $T_c$/8, for
current directions perpendicular ($J \parallel a$) and parallel ($J \parallel
c$) to the tetragonal c axis. For $J \parallel a$, a sizable residual linear
term $\kappa_0 / T$ is observed, as previously, which confirms the presence of
line nodes in the superconducting gap. For $J \parallel c$, on the other hand,
$\kappa / T \to 0$ as $T \to 0$. The resulting precipitous decline in the
anisotropy ratio $\kappa_c / \kappa_a$ at low temperature rules out a gap
structure with line nodes running along the c-axis, such as the d-wave state
favoured for CeCoIn$_5$, and instead points to a hybrid gap of $E_g$ symmetry.
It therefore appears that two distinct superconducting states are realized in
the Ce$M$In$_5$ family.
