  We have investigated the bulk and microscopic properties of the rhombohedral
intermediate valence superconductor CeIr$_3$ by employing magnetization, heat
capacity, and muon spin rotation and relaxation ($\mu$SR) measurements. The
magnetic susceptibility indicates bulk superconductivity below $T_\mathrm{C} =
3.1$~K. Heat capacity data also reveal a bulk superconducting transition at
$T_\mathrm{C} = 3.1$~K with a second weak anomaly near 1.6~K. At
$T_{\mathrm{C}}$, the jump in heat capacity $\Delta C$/$\gamma T_{\mathrm{C}}
\sim 1.39(1)$, is slightly less than the BCS weak coupling limit of 1.43.
Transverse-field $\mu$SR measurements suggest a fully gapped, isotropic,
$s$-wave superconductivity with 2$\Delta(0)/k_{\mathrm{B}}T_{\mathrm{C}} =
3.76(3)$, very close to 3.56, the BCS gap value for weak-coupling
superconductors. From the temperature variation of magnetic penetration depth,
we have also determined the London penetration depth $\lambda_{\mathrm{L}}(0) =
435(2)$~nm, the carriers' effective mass enhancement $m^{*} =
1.69(1)m_{\mathrm{e}}$ and the superconducting carrier density $n_{\mathrm{s}}
= 2.5(1)\times 10^{26}$ carriers m$^{-3}$. The fact that LaIr$_3$, with no
$4f$-electrons, and CeIr$_3$ with $4f^{n}$ electrons where $n \le 1$-electron
(Ce ion in a valence fluctuating state), both exhibit the same $s$-wave gap
symmetry indicates that the physics of these two compounds is governed by the
Ir-$d$ band near the Fermi-level, which is in agreement with previous band
structure calculations.
