Improving the robustness, vis-a-vis matched input disturbances of interconnection and damping assignment, passivity based control (IDA-PBC) for a class of underactuated mechanical systems is addressed in this paper. The characterised class of systems is described by a Port Controlled Hamiltonian (PCH) model which represents another alternative to the classical Euler-Lagrange models for which IDA-PBC yields a smooth stabilising controller. Our main contribution consists of combining the so-called IDA-PBC controller with an adaptive control technique. Some sufficient stability conditions on matched input disturbances are given. In order to estimate the stability and performance robustness of both controllers, a stochastic robustness analysis was used. Indeed, we used the Monte Carlo simulation (MCS) based on uncertainties to analyse the behaviour of the closed-loop system. The comparison of the stability robustness between the classical IDA-PBC controller and the proposed one is then provided. As an illustration, we proposed to revisit the application of IDA-PBC controller to the inertia wheel inverted pendulum in the presence of matched disturbances. Simulation and real-time experimental results mirror the theoretical results and prove the efficiency of the proposed controller.