Accessibility and transport provision have played a key role in shaping the development patterns of major urban areas internationally. While future urban development patterns are subject to fluctuations of the economy and resulting resource decisions we can at a minimum explore potential likely future development scenarios based on current evidence and projections. In this study developed with reference to published comparable international research case studies, the environmental sustainability implications of planned rapid rail infrastructure investments on the urban form in the Greater Dublin Area (GDA) have been analysed incorporating the scenario analysis approach. Various scenarios are generated by the MOLAND Model (which simulates alternative development conditions by using historical land use datasets through the utilisation of cellular modelling) applications including: A baseline scenario incorporating a continuation dispersed pattern of urban development, and an alternative scenario with rapid rail-oriented corridor development. These scenarios are subsequently examined. In terms of sustainable urban development considerations, dispersed development in the baseline case indicates costs of such development exceed the benefits. In contrast, the containment policies -as in the public transport-oriented development could achieve benefits over the baseline scenario by reducing the negative consequences of sprawled type of developments. Further explorations incorporating a Cost- Benefit Analysis (CBA) approach is developed to evaluate the sustainability implications of the two different land development scenarios in the Dublin Area. This is assisted by giving emphasis to the impacts of rapid rail investments on urban form and development as raised in the international comparative literature.