The objective of this paper was to investigate the options for the sustainable development of the Northern Ireland energy supply system, reducing greenhouse gas emissions. Particular factors which must be considered centre around the isolation of the system, the prospect of a new gaspipeline and electrical interconnector to the rest of the U.K., and the re-establishment of an electrical interconnector to the Republic of Ireland. The study has relevance for all similar isolated or island systems, which have particular problems because they tend to be relatively small in size, and their isolation means that inter-connection with their neighbours is either weak or non-existent. Because of this, they have to carry greater spare capacity than would normally be the case, with consequent efficiency and cost penalties.