Poly- and perfluoroalkyl substances(PFAS) are a large group of compounds consisting of a partially or fully fluorinated carbon chain. PFAS are used across industries including electronic and pharmaceutical manufacturing, and in firefighting foams. PFAS are now ubiquitous in the environment showing bioaccumulation and adverse health effects in wildlife and humans. Little information on PFAS concentration in Ireland’s marine environment exists. The 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). Extracts were analysed by High-Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS) using a Zorbax Eclipse Plus C18 column( 3.0 x 50 mm; 18µm). 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 decreased from upper to lower stream sites with a concentration of 57.7 ng/l at the final sampling site in Dublin Bay. 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. Two sampling sites were located upstream and downstream of Ringsend wastewater treatment plant (WWTP). Five analytes detected here were found to increase downstream of the WWTP, including GenX which increased from 18.8 to 23.9 ng/l, and perfluorohexanoic acid 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 sites.