The cathedral city of Armagh is one of the most historically and architecturally significant on the island of Ireland. This article explores the preparation of an inventory of Armagh’s architectural heritage by the London architect-planner Max Lock in 1964, commissioned by the Northern Ireland Committee of the National Trust. The inventory represents one aspect of the initial response of civil society to impending change in the built environment in the mid-1960s and formed part of efforts to ensure parity with Great Britain in land-use planning legislation. The inventory facilitates a wider discussion on state-civil society relations in Northern Ireland, the values and ideas guiding change within historic settlements, and the place promotional advantage that the local council sought to derive from Armagh’s history and heritage. Utilizing Ward’s typology of diffusion, it is argued that Northern Ireland represents an unusual ‘within-UK’ example of the transference of planning ideas and practices, with its historical experience of devolution offering valuable contemporary insights into the increasingly diffuse and fragmented governance space within the UK.