During the 1990s the Celtic Tiger era began in the Republic of Ireland. This article tracks the response of the Irish Urban System to that remarkable period of growth ended abruptly with the Global Economic Crisis of 2008. Using Small Area Population Statistics from Ireland’s Central Statistics Office for the years 1996, 2002, 2006 and 2011 it was possible to record growth across the towns and cities of Ireland that constituted the Irish Urban System. The location, size, type and rates of change were recorded and mapped with a view towards discovering the extent to which the urban hierarchy and the spatial distribution was being altered, and by what geographical processes. Over 15 years the national population grew by 26% with most of that growth taking place in urban centres. A clear diffusion outwards from the Dublin region is noticeable and the capital’s role in systemic change is explored alongside other factors. The article highlights the changing nature of growth over time and, based on the empirical observations made, identifies a sequence of clear stages in the growth of the urban system. The article concludes with a proposal for a Model of Urban System Evolution under conditions of Rapid Economic Growth based on the distinct phases, or stages, of growth identified in Ireland’s towns and cities from 1996–2011.