Radioactive effluent has been discharged from the UK Sellafield nuclear reprocessing facility to the Eastern Irish Sea since 1952. Waste storage ponds and process liquors from spent fuel reprocessing activities are the main source of this effluent, and contain actinides and fission products (Am, Pu, Np, U, Tc, Cs). These radionuclides accumulate in finely-grained sediments in the Irish Sea and the onset of various biological and physical processes results in the resuspension of radionuclide-labelled sediments. Tidal and bottom currents then transport these radionuclides to neighbouring estuarine regions e.g. Ravenglass saltmarsh. The examination of radionuclide biogeochemistry in this environment provides a long-term (+65 years) assessment of radionuclide behaviour. To date, the majority of studies conducted with Sellafield-contaminated materials have focused on ascertaining radionuclide distribution profiles and matching these to discharge histories, where available. The importance of potential biogeochemical controls on actinide transport and eventual fate has not been studied in great detail. Here, we have used ion-exchange separation techniques coupled with radiometric analysis, HR-MC-ICP-MS, and AMS to determine contemporary actinide (241Am, 236U, 237Np, 238,239,240Pu, 241Pu) and fission product (137Cs and 99Tc) distribution at two marine sites impacted by the Sellafield discharges (the Irish Sea mud-patch and Ravenglass saltmarsh). Elemental analysis of the sediments was also completed via ICP-AES and XRF techniques. The results suggest that the distribution and long-term fate of 137Cs, 241Am, and the Pu isotopes at the Ravenglass saltmarsh is largely independent of the ambient redox-related processes, and is instead controlled by the physical transport and burial of the time-integrated Sellafield discharge history. Plutonium in the laboratory has previously been shown to be influenced by trace metal cycling, however this was not observed here. The radionuclides, 99Tc, 236U, and 237Np at the Ravenglass saltmarsh did not reflect their discharge histories, and instead it is suggested that the redox-cycling of these radionuclides likely occurs, driven by the favourable sub-oxic conditions of the sediments, leading to post-depositional remobilisation. This is consistent with traditionally held views garnered from constrained laboratory studies, suggesting similarity between the real world and the laboratory. Sequential extraction experiments show Am and Pu to be primarily associated with the refractory sediment phases, and thus are not environmentally available, with a fraction of Cs present in the easily exchangeable fraction, consistent with the known preferential association of Cs with surfaces of clay minerals.