The infrared spectrum of a monolayer of CO physisorbed on the NaCl(100) surface has been calculated using the discrete dipole approach. The corresponding description is rigorously microscopic and accounts exactly for the occurring local fields in both the substrate and the adsorbed monolayer. The model does not impose restrictions upon the configuration and links smoothly the microscopy of the system to the externally observable macroscopic response. Using established values for the geometry and polarizabilities of the NaCl, we have calculated the integrated absorbance of the system. We find that, in order to arrive at an agreement between theory and experiment, the usual values for the polarizability and/or the height above the surface of the CO molecule should be decreased. No unique answer as to the value of these two parameters can be obtained from interpreting IR absorption experiments.