The initial dissociative chemisorption probability, S-0, of O-2 on Ir(111) has been investigated with molecular beam techniques and electron energy loss spectroscopy (EELS). The adsorption process on the clean surface occurs by distinct dynamical mechanisms. At incident kinetic energies, E-i, of 0.1 eV and below, the dissociative chemisorption probability decreases with increasing kinetic energy, indicating the dominance of a trapping-mediated mechanism. A decrease in the value of S-0 with increasing surface temperature, T-s, is also characteristic of this regime. This temperature dependence reflects the participation of a physically adsorbed state and molecularly chemisorbed state in the dissociation scheme. Additionally, the dependence of S-0 on incident angle, theta(i), in the low kinetic energy regime exhibits near normal energy scaling. At high kinetic energy (E-i>0.1 eV), the initial dissociative chemisorption probability rises with increasing E-i indicating that translational energy is effective in surmounting a potential barrier to adsorption. Direct access of a molecularly chemisorbed state followed by dissociation, rather than direct access of the dissociated state, is hypothesized to be the primary initial adsorption step. Several observations support this mechanism, including a temperature dependence in the high kinetic energy regime and no observed increase in oxygen saturation coverage with increasing kinetic energy. In addition, EEL spectra show that molecularly chemisorbed states of oxygen are formed on the Ir(111) surface at T-s<70 K after exposure to a 1.3 eV beam and partial saturation of the atomic overlayer. Attempts to identify molecularly chemisorbed oxygen at low coverages were unsuccessful and limited by the experimental setup which provides cooling of the iridium crystal to only similar to 68 K.