We present a general treatment of the optical response of a monolayer physisorbed on a thick dielectric slab. The description, based on the discrete-dipole approach, is rigorously microscopic and accounts exactly for the occurring local fields in both the substrate and the monolayer. The model links smoothly the microscopy of the system to the externally observable macroscopic response. Within this theory we treat the optical response in the infrared of a monolayer of CO physisorbed on the NaCl(100) surface. Using established values for the geometry and polarizabilities of the NaCl, we calculate the integrated absorbance of the system. We find that, in order to arrive at an agreement between theory and experiment, one can either decrease the distance between the CO monolayer and the NaCl surface with respect to the values usually quoted in literature and/or decrease the vibrational polarizability of CO with respect to its gas-phase value. No unique answer as to the value of these two parameters can be given by just interpreting ir-absorption experiments.