On 23 April 2006, an ethnicity question appeared for the first time on the census in the Republic of Ireland. This article analyses the evolution and addition of this question as an illustration of a specific process of state racialization in the Irish census. As such, it illuminates the social and political contestation of the meaning of race, racial categories and ethnicity in the Republic of Ireland through an examination of the interplay between demographers' needs for simple categorization and the complex lived reality of race and ethnicity in Ireland. Driven by the `Celtic Tiger' economic boom and reversing the historic trend of Irish emigration, immigration has increased to levels not generally seen before 1996 in Ireland. The article shows how a growing diverse population of immigrants to Ireland, an increased awareness of equality legislation and a need to rationalize the statistical systems in Ireland all created a desire to enumerate ethnic groups. The article also explores how the Irish census arrived at the particular form of racial and ethnic categorization that it did — influenced by international censuses (particularly from the UK with which it shares a common travel area), the historical ethnicization of Travellers (as the article shows, there has been a long-standing debate about whether Travellers, a disadvantaged indigenous nomadic group, are considered `ethnic' or not) and increasing awareness of ethnocultural characteristics among European statistical agencies.