This study, based on 2019 data, investigates the cost-effectiveness of Shore Side Electricity (SSE) adoption uti- lising the existing and future (2030) Irish energy mix, while considering different levels of adoption across six scenarios, incorporating both “port-side” and “ship-side” private costs alongside the benefits from reduced external costs and avoided Auxiliary Engine (AE) fuel costs. Passenger ships calling to the two most populated Irish port cities of Dublin and Belfast were selected as the case study, owing to the significance of such ports and ship types in the Irish maritime economy. For the existing Irish energy mix, the most optimal solution among the discussed scenarios was that of switching the top 10 most frequent passenger ship callers in Dublin and Belfast to SSE, as it reflected the highest Net Present Values (NPV) of €34.06 million and €15.44 million, respectively. The future (year 2030) SSE supply is expected to be “cleaner” due to an increase in the uptake of renewable energy sources, which will further boost the obtained NPVs by 50%. A combination of public funding (by 50%), increment in supplied electricity price by 8.62% (for Dublin) and 10.01% (for Belfast) and an annual ticket price supplement (per passenger seat) by €0.03 (for Dublin) and €0.04 (for Belfast), can create a business case for the identified optimal scenario.