The intrinsic spatial nature of development plans poses specific requirements on the analytical tools applied to support Strategic Environmental Assessment (SEA) processes. Geographic information systems (GIS), with their mapping and analytical potential, can assist and enhance the various stages of SEA. A method has been developed to apply GIS as a support tool to assist SEA of land use plans in the Republic of Ireland. This chapter describes one phase in the development and testing of the method during the preparation of County Development Plans, a participatory Internet-based GIS tool developed to communicate and gather information in a spatially specific format. The aim of the web site was to promote and expand the use of GIS in public participation and, thus, allow for the incorporation of spatially specific public perceptions in SEA. The results revealed that the integration of public perceptions into the assessment through GIS stimulates debate and provides an overall scientific and social view of the relative environmental significance and vulnerability of the different areas. However, current issues in relation to availability and quality of spatial data constrained the applicability of GIS. Furthermore, complexity of the technology, data disclosure issues, and statutory consultation requirements restricted its implementation and use, affecting the adequacy and the level of public opinion gathered through the Web site.