The literature review presented in this chapter centres on the external factors that influence PV output performance in the Irish climate. Solar photovoltaic (PV) panels show long-term performance degradation, resulting in lower like-per-like efficiencies and performance ratios. Manufacturers of solar photovoltaic modules typically guarantee a life span of more than 20 years. But to meet such guarantees, it is necessary to track and mitigate PV module degradation during this period, and identify maintenance and repair requirements beyond this period. Solar PV modules degrade over time, becoming less efficient, less reliable and, eventually, inoperable. External factors such as solar irradiance, dust deposition, shading, ambient temperature, operating cell temperature, humidity and wind velocity affect the PV output performance in the Irish climate. This is because the performance of a PV system is heavily influenced by the meteorological conditions of the site locations. Therefore, this chapter reviews the external factors that influence PV output performance in the Irish climate.