This paper describes a power system inertia estimation method that utilizes power perturbations caused by the flow switch on an HVDC interconnector. The analysis is performed using real power system measurements from Ireland's all-island power system. The method employs an energy based estimation technique which permits a non-step power change to be used for inertia calculation and encapsulates the challenges associated with the use of real data. While it is shown that synchronous generators account for most of the power system inertia, other contributors are also investigated, namely power station auxiliary load, distributed generation and power system demand. Time-of-day and wind related effects on Ireland's power system inertia are explored. Recommendations are made for the practical implementation of the method.