Ecclesiastical dispensations from the impediment of consanguinity were studied in relation to 190,557 marriages between two Catholics in all Ireland for the ten-year period 1959–1968. The data, based on approximately 71% of all marriages in this period, indicate a first cousin marriage rate of about 1 in 720 for all Ireland. Figures for the Republic of Ireland, based on approximately 91% of all marriages, indicate a first cousin marriage rate of about 1 in 625 for this section of the country. Various aspects of the data, including ‘isolate’ effects and the influence of itinerancy, are discussed.