While ethno-national demography has featured significantly in political and scholarly debates about Northern Ireland, little attention has been paid to the politics of the Northern Ireland census itself. This article addresses this gap by exploring census politics since the 1998 Good Friday Agreement. It demonstrates that the border poll provision of the Good Friday Agreement and Brexit have increased the political salience of census results, with the 2021 census now being anticipated by many as a potential referendum trigger. Against this background, I argue that new census questions – on religious background and national identity – have had significant consequences for debates about the constitutional future. Introduced in order to satisfy requirements stemming partly from equalities legislation, an unintended consequence of the religious background question has been to reinforce ‘two communities’ narratives in constitutional debates, whereas the national identity question has served to problematise assumptions about relationships between identity and constitutional preferences.