This paper describes the potential environmental changes caused by tidal power installations with illustration for schemes in the eastern Irish Sea, focussing mainly on major estuarine barrages. The generic impacts in the near-field and far-field are discussed. Results from a 0-D and a 2-D model are presented: the former allows rapid calculations to be made for a large range of options while the latter allows the full effect on 2D hydrodynamics to be investigated. It is shown that there may be a significant change in tidal amplitude at the coast of Northern Ireland. The bed stress in the Bristol Channel will be significantly reduced if a Severn barrage is constructed. Some effects on the tidal mixing are expected although the location of tidal fronts in the Irish and Celtic Seas will not be changed significantly. The largest environmental impact is expected to be on the amount of inter-tidal area retained after construction of an estuarine barrage. It is shown that the loss of mudflats can be substantially reduced by using a dual-mode (ebb and flood generation) scheme with an increased number of turbines over the lowest-cost option.