We have investigated the superconducting state of HfIrSi using magnetization, specific heat, muon spin rotation and relaxation (SR) measurements. Superconductivity was observed at K in both specific heat and magnetization measurements. From an analysis of the transverse-field SR data, it is clear that the temperature variation of superfluid density is well fitted by an isotropic Bardeen?Cooper?Schrieffer (BCS) type s-wave gap structure. The superconducting carrier density m(?3), the magnetic penetration depth, nm, and the effective mass, , were calculated from the TF-SR data. Zero-field SR data for HfIrSi reveal the absence of any spontaneous magnetic moments below , indicating that time-reversal symmetry (TRS) is preserved in the superconducting state of HfIrSi. Theoretical investigations suggest that the Hf and Ir atoms hybridize strongly along the c-axis, and that this is responsible for the strong three-dimensionality of this system which screens the Coulomb interaction. As a result, despite the presence of d-electrons in HfIrSi, these correlation effects are weakened, making the electron-phonon coupling more important.