An analytical formula was proposed recently to predict the accurate P-branch spectral lines of rovibrational transitions for diatomic systems by taking multiple spectral differences. A similar analytical expression was suggested here to predict the Q-branch spectral lines of rovibrational transitions. This formula was applied to study the high-lying Q-branch emission spectra of the (4,1) and (3,1) bands of the A1Π—X1Σ+ system of IrN molecule using fifteen known accurate experimental transition data. The results show that not only the known experimental transition lines were reproduced but also the correct values of the unknown spectral lines were predicted.