  We present the fully relativistic multi-reference configuration interaction
calculations of the ground and low-lying excited electronic states of IrO for
individual spin-orbit component. The lowest states for four spin-orbit
components 1/2, 3/2, 5/2, and 7/2 are calculated intensively to clarify the
ground state of IrO. Our calculation suggests that the ground state is of 1/2
spin-orbit component, which is highly mixed with $^4\Sigma^-$ and $^2\Pi$
states in $\Lambda-S$ notation. The two low-lying states of the 5/2 and 7/2
spin-orbit components are nearly degenerate with the ground state and locate
only 234 and 260 cm$^{-1}$ above, respectively. The equilibrium bond length
1.712 \AA \ and harmonic vibrational frequency 903 cm$^{-1}$ of the 5/2
spin-orbit component are close to the experimental measurement of 1.724 \AA \
and 909 cm$^{-1}$, which suggests the 5/2 state should be the low-lying state
contributed to spectra in experimental study. Moreover, the electronic states
that give rise to the observed transition bands are assigned in terms of the
excited energies and oscillator strengths obtained for the 5/2 and 7/2
spin-orbit components.

Chemical Physics