A pseudomorphic monolayer of Au on Ir(001) is easily preparable and has a long lifetime in ultrahigh vacuum. Its geometrical and electronic structure was investigated by ab initio calculations within density functional theory. For a wide range of energies (below 100 eV) and angles of incidence of spin-polarized primary electrons we measured and calculated the intensity and spin asymmetry of the specularly reflected beam. In the resulting energy-angular maps, which show good agreement between experiment and theory, we identify several regions of high asymmetry and sizable intensity. These regions, together with the long lifetime in ultrahigh vacuum, make pseudomorphic Au on Ir(001) very suitable as a two-dimensional spin-polarizing mirror in momentum microscopes and behind dispersive electron energy analyzers.