The transition from a fossil-fuel to a renewable energy system is a modern and complicated challenge for numerous countries. However, as Ireland is an island that is poorly interconnected to other energy systems, this challenge becomes even more complicated. Identifying how to make this transition is a vital step due to the scale of the change required for large-scale renewable penetrations. In this paper, a model of the Irish energy system is created to identify how Ireland can transform from a fossil-fuel to a renewable energy system. The energy-systems-analysis model, EnergyPLAN, was chosen to create the model as it accounts for all sectors that need to be considered for integrating large penetrations of renewable energy: the electricity, heat and transport sectors. Before various alternative energy-systems could be investigated for Ireland, a reference model of the existing structure needed to be created. This paper focuses on the construction of this reference model, in terms of the data gathered, the assumptions made and the accuracy achieved. In future work, this model will be used to investigate alternative energy-systems for Ireland, with the aim to determine the most effective energy system for integrating significant quantities of renewable energy.