The IR absorption peak position of the CO intramolecular stretching vibration, measured with Fourier transform infrared reflection-absorption spectroscopy, has been employed for the first time as a nonintrusive coverage probe to study the adsorption of CO on Ir(111). The observed temperature and coverage dependence of the adsorption probability is consistent with an extrinsic precursor model. Isobars were obtained under steady-state conditions for pressures ranging from 10(-9) to 10(-5) Torr. Isosteres were constructed from these data and used to determine both isosteric heats of adsorption and the adsorbate entropy as functions of coverage. The isosteric heat of adsorption decreased from 44 +/- 2 kcal/mol at 0.05 ML to 37 +/- 1 kcal/mol at 0.6 ML. A pronounced decrease in the binding energy at 0.35 ML coincides with the weakening of the (root 3 x root 3)R30 degrees LEED pattern and is attributed to near-neighbor CO repulsions. The entropy of the adsorbate layer indicates two ordering regions which correspond to the formation of the (root 3 x root 3)R30 degrees and (2 root 3 x 2 root 3)R30 degrees overlayer structures.