Poly- and perfluoroalkyl substances (PFAS) are a large group of compounds consisting of a partially or fully fluorinated carbon chain. Due to their physio-chemical properties these substances are widely used across a range of industries including electronic and pharmaceutical manufacturing as well as use in firefighting foams. As a result of growing use and their stable nature, PFAS are now ubiquitous in the environment showing bioaccumulation and adverse health effects in wildlife and humans. Although PFAS are currently being studied worldwide, little information on PFAS concentration in Ireland’s marine environment exists. The main objective of this study was to determine PFAS concentration in transitional and marine waters along Dublin Bay and pinpoint areas where the existing concentration of these compounds, or slight increases could pose an environmental problem. Marine samples were treated with sodium thiosulfate to reduce free chlorine prior to analysis. Water samples were extracted by solid phase extraction (SPE) using Oasis WAX (500 mg). The extract was analysed by High performance liquid chromatography (HPLC) using an Agilent Zorbax Eclipse Plus C18 column (3.0 x 50 mm; 18µm) and an identical guard column. For detection, HPLC was coupled with a triple quadrupole mass spectrometer. Results confirm the presence of both legacy and novel PFAS in transitional waters entering Dublin Bay, with fluctuations in concentrations observed in upper and lower estuary sites along the river Liffey. The highest concentrations detected were 1393.7 and 1217.9 ng/l at upper estuary sites for perfluorooctanoate (PFOA) and perfluoroundecanoate (PFUdA) respectively. Levels of PFOA, a legacy PFAS, decreased from upper to lower stream sites with a concentration of 57.7 ng/l at the final sampling site in Dublin Bay. The novel PFAS perfluoropropoxypropanoic acid (GenX) was detected at upper estuary sites with the highest concentration of 37.8 ng/l, this decreased downstream at lower estuary sites to <25 ng/l. The two final sampling sites were located upstream and downstream of Ringsend wastewater treatment plant (WWTP). Five of the seven analytes detected at these two sites were found to increase downstream of the WWTP, these analytes include GenX which increased from 18.8 to 23.9 ng/l, and perfluorohexanoic acid (PFHxA) which increased from 95.8 to 157.9 ng/l downstream of the WWTP. PFUdA concentration was found to decrease from 1066.3 to 582.7 ng/l between these two sites.