The adsorption and dissociation of dioxygen on the Ir(111) surface have been studied using periodic self-consistent density functional theory (DFT-GGA) calculations. Three di-sigma-type chemisorbed molecular precursors are identified: One is located over bridge sites and has a binding energy of -1.3 eV and a bond length of 1.4 A; the other two are located over threefold hollows and have similar binding energies of ca. -1.2 eV and bond lengths of ca. 1.5 A. None are magnetic, suggesting a peroxo (O-2(2-)) nature for these precursors. The minimum energy path for O-2 dissociation is determined using the nudged elastic band method, and a very small activation energy is found, ca. 0.06 eV.