We have reported first principles calculations based on the density functional theory within the local density approximation to explain the structural, electronic, elastic, vibration (phonon dispersion curves and one-phonon density of states) and optical properties for NbIrSn, which is semiconductor half Heusler compound with a cubic MgAgAs type structure. The obtained results are compared with the available other theoretical data. Our band structure calculations show that NbIrSn has indirect energy band gap of similar to 1 eV at Gamma-X Brillouin zone. The elastic properties, namely, bulk modulus, elastic constants, shear modulus, Young's modulus, Poisson's ratio, lame constants and Kleinmann parameter, are calculated under high pressure. The pressure effects for optical properties including the real and imaginary parts of the complex dielectric constant, refractive index n(omega), extinction coefficient k(omega), energy loss function L(omega) and reflectivity R(omega) are also computed and discussed.