Thresholds for environmental impact assessment, EIA, for specified classes of development are set out in the First Schedule to the European Communities (Environmental Impact Assessment) (Amendment) Regulations, 1999 (S.I. No. 93 of 1999).
Under the regulations, EIA is mandatory for the following development proposals. Installations for hydro-electric energy production with an output of 20 megawatts or more, or where the new or extended superficial area of water impounded would be 30 hectares or more, or where there would be a 30% change in the maximum, minimum or mean flows in the main river channel; construction of overhead electrical power lines with a voltage of 220 kV or more and a length of more than 15 km
Where a planning authority considers that a development proposal which is below the specified threshold is likely to have significant environmental effects, it is obliged to request an EIS from the developer.