Both the time domain simulations based on potential flow theory and wave tank model tests are used to evaluate the motion responses of a wave energy device. In this work, system identification is applied to obtain the frequency dependent transfer functions (between motions and excitations) from a series of model test runs of a wave energy platform exposed to irregular (random) waves. This is the first application of Reverse-Multiple Input Single Output (R-MISO) to a realistically (catenary) moored system (typical characteristics of wave energy devices) comparing physical model tests with our in-house time domain simulation program with addition of a mooring model. Based on the comparisons between the transfer functions from the time domain simulations and those from the model tests, reasonable frequency dependent dampings have been directly pulled out from the test cases under random sea states. System identification derived corrections to the linear or quadratic damping in pitch significantly improved the accuracy of motion responses. In this sense, this methodology can be a powerful tool in assisting the accurate simulation and design of wave energy devices under random sea states.