Per- and polyfluoroalkyl substances (PFAS) are a large family of widely used legacy and emerging contaminants that are pervasive in aquatic habitats globally. These molecules are chemically very stable and are highly resistant to biological degradation; therefore, they belong to a class of compounds that tend to persist in the environment and potentially may present a risk to the health of humans and wildlife. In Ireland, data on the occurrence of these chemicals in freshwater ecosystems is scarce. In order to assess the threat PFAS pose for the environment and public health, this study had the objective of evaluating the occurrence, levels and distribution of fifteen PFAS, both legacy and emerging, in the Liffey river in the city of Dublin. The Liffey flows through agricultural land and into a large urban area, and the sampling points selected contained multiple diffuse and point sources. Analytes were extracted by solid phase extraction (SPE) and determined by liquid chromatography triple quadrupole mass spectrometer (LC-QqQ-MS). Target compounds included 8 perfluorocarboxylates (PFCAs) C5–C12, 7 perfluorosulfonates (PFSs) C4–C10 and 1 perfluorosulfonamide (FOSA), C8. The quality parameters for the fifteen selected compounds presented good limits of detection (LOD) ranging, in general, from 0.56–12.6 ng/L. The method was applied to assess the occurrence of PFAS in 80 river water samples over four sampling campaigns, accounting for seasonal variation. Ten out of fifteen compounds were consistently detected in the majority of the samples, at levels that range from 1 to 400 ng/L. The study offers an overview of the correlation between use land and PFAS occurrence in a major city.