Spin-orbit torque (SOT) offers promising approaches to developing energy-efficient memory devices by electric switching of magnetization. Comparing to other SOT materials such as heavy metal and topological insulator, metallic antiferromagnet (AFM) potentially influences the generation of SOT through its magnetic structure. Here, combining the results from neutron diffraction and spin-torque ferromagnetic resonance experiments, we show that the room-temperature magnetic structure of epitaxially grown L10-IrMn (a collinear AFM) is distinct from the widely presumed bulk one. It consists of two types of domains with the spin axes orienting towards  and [-111], respectively. We find that this unconventional magnetic structure is responsible for a much larger SOT efficiency up to 0.60±0.04, comparing to 0.083±0.002 for the polycrystalline face-centered-cubic IrMn. Furthermore, we reveal that the magnetic structure of L10-IrMn induces a large isotropic bulk contribution to the SOT efficiency and an anisotropic interfacial contribution of comparable magnitude, where the latter depends strongly on the electric current direction in the film plane. Our findings shed light on the critical roles of bulk and interfacial antiferromagnetism to SOT generated by metallic AFM.