Abstract: The typically nocturnal, lunar and seasonal activity cycles of European eel were documented during commercial eel fishery monitoring and eel population surveys in Ireland. The relationship of eel activity cycles to light levels were observed in all continental eel life-history stages (glass eel / elver, yellow-phase and silver-phase) and examples of these behaviour patterns will be presented. Anthropogenic light sources have also been shown to affect eel swimming activity and results of light deflection experiments undertaken in the River Shannon at Killaoe eel fishing weir during the 2014/2015 migration period will be described. The eel fishing weir is located immediately downstream of an old stone road bridge that crosses the river. LED lights, mounted on the fishing weir, were directed upstream though the bridge’s navigation arch, to which the fastest flowing water was diagonally (SE) orientated. Operation of the lights resulted in catches in nets at the navigation arch declining (as % of the total weir catch) from 30% (lunar dark peak) and 14% (mean during full lunar month) to approximately 5%. Eel numbers also passed downstream via the western bridge arches to the fishing net arrays downstream of them. This observation is consistent with the northerly direction of the light beams relative to the generally SE orientation of the illuminated water moving to the navigation arch. The potential use of eel fishing weirs such as the one at Killaloe for assessment of potential measures for mitigation of effects of hydropower dams on silver-phase eels will be discussed.