The Irish urban system has evolved over time. Urban centres have moved up and down the urban hierarchy, and the delineation of urban spheres of influence for the top levels of the urban hierarchy in 1966, 1971 and 1981 indicates that hinterlands have also changed. New centres in the upper tiers of the hierarchy developed in areas where the hinterlands of the older centres in that tier had previously converged. The analysis of such spheres of influences and urban attraction over time can be used as an important aid for a variety of types of government planning.