  We report the temperature- and magnetic-field-dependent optical conductivity
spectra of the heavy electron metal YbIr$_2$Si$_2$. Upon cooling below the
Kondo temperature ($T_{\rm K}$), we observed a typical charge dynamics that is
expected for a formation of a coherent heavy quasiparticle state. We obtained a
good fitting of the Drude weight of the heavy quasiparticles by applying a
modified Drude formula with a photon energy dependence of the quasiparticle
scattering rate that shows a similar power-law behavior as the temperature
dependence of the electrical resistivity. By applying a magnetic field of 6T
below $T_{\rm K}$, we found a weakening of the effective dynamical mass
enhancement by about 12% in agreement with the expected decrease of the
$4f$-conduction electron hybridization on magnetic field.
