We investigate the spin-orbit torque in a Ta/Ir−Mn/Cu/Ni−Fe multilayer heterostructure and relate it to spin current transmission through the Ir−Mn layer. We identify several spin current transport regimes as a function of the temperature and the thickness of the Ir−Mn layer. To interpret this experiment, we develope a drift-diffusion model accounting for both electron and magnon transport in the heterostructures. This model allows us to discriminate between the contributions of electrons and magnons to the total spin current in Ir−Mn. We find that the electron-magnon spin convertance is one order of magnitude larger than the interfacial electronic spin conductance, while the magnon diffusion length is about ten times longer than the electronic spin relaxation length. This study demonstrates that magnonic spin transport dominates over electronic spin transport even in disorder metallic antiferromagnets.