This paper investigates the relationships between settlement size, functionality, geographic location and sustainable development. Analysis was carried out on a sample of 79 Irish settlements, located in three regional clusters. Two methods were selected to model the level of sustainability achieved in settlements, namely, Metabolism Accounting and Modelling of Material and Energy Flows (MA) and Sustainable Development Index Modelling. MA is a systematic assessment of the flows and stocks of material within a system defined in space and time. The metabolism of most settlements is essentially linear, with resources flowing through the urban system. The objective of this research on material and energy flows was to provide information that might aid in the development of a more circular pattern of urban metabolism, vital to sustainable development. In addition to MA, a set of forty indicators were identified and developed. These target important aspects of sustainable development: transport, environmental quality, equity and quality of life issues. Sustainability indices were derived through aggregation of indicators to measure dimensions of sustainable development. Similar relationships between settlement attributes and sustainability were found following both methods, and these were subsequently integrated to provide a single measure. Analysis identified those attributes of settlements preventing, impeding or promoting progress towards sustainability.