The <i>Northern Ireland Grid-Square Product</i> comprises population counts for a regular grid of 1km and 100m cells for 1971, 1981, 1991 and 2001. It is a unique data resource, which has previously been under-utilised because it has been held in an inaccessible format, where data have had to be extracted by the manual entry of each cell's co-ordinates. The data are valuable because they permit the analysis of change through time using geographical units that are consistent between censuses. The <i>Linking Northern Ireland Census of Population Data</i> project sought to make these data more widely available by extracting all counts for all years and subsequently reformatting the data to make them more amenable for general analysis with commonly used software. Linkages between variables through time have been documented, and basic look-up tables linking the 1km and 100m cell counts to <i>Census 2001</i> output geographies have been provided. All variable counts for all years are given. The data make it possible to trace local patterns of social, demographic and economic change in Northern Ireland through time, using a consistent geography between 1971 and 2001.<br>