Fish and jellyfish interact in a range of ways, many of which contribute to fish stock success. Jellyfish can compete with fish for food resources, or feed on fish eggs and larvae. They also provide habitat and space for developing larval and juvenile fish which use their hosts as means of protection from predators and feeding opportunities. Furthermore, the form of these complex interactions are likely highly temporally and spatially variable. Yet, the broader ecological relevance of jellyfish is often neglected beyond their role as stressors to the marine environment, including fish communities. This is a gross over-simplification of the true role of jellyfish in the marine environment, yet the link between field biologists and modellers remains a limiting factor in their inclusion in ecosystem and fisheries models. Using a multidisciplinary approach, this thesis aimed to provide a balanced understanding of the net impact of jellyfish on fish communities in the Irish Sea, and to provide a means for researchers to do the same in marine systems around the world.