The chemisorption of atomic (H, O, N,,S, C), molecular (N-2, CO, NO, NH3) and radical (CH3, OH, NOH) species on Ir(111) has been systematically studied. Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using PW91 and RPBE functionals, have been used to determine preferred binding sites, chemisorbed structures, binding energies, vibrational frequencies, and the effect of surface relaxation for the above species at 0.25 ML surface coverage. The following order in binding energies from least to most strongly bound was determined: N-2 < NH3 < NO < CH3 < CO < OH < H < NOH < O < N < S < C. A preference for 3-fold sites for the atomic adsorbates was observed, with the exception of atomic H, which prefers top sites. Molecular species showed a preference for top sites with the exception of NOH; this species preferred fcc sites. Surface relaxation had only a small effect on energetics in most cases. Calculated vibrational frequencies, in general, were in good agreement with experimental frequencies and support experimentally proposed site preferences for all adsorbates where data are available. Finally, the thermochemistry of CO, NO, NOH, N-2, NH3, and CH3 decomposition on Ir(111) was examined, leading to predictions for each species of the preference for either desorption or decomposition.