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.
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