Based on a sample of about 5,000 members, this article offers the first comprehensive social profile of the IRA covering the entire period of the Irish revolution. The picture that emerges is of an organization composed largely of unpropertied, unmarried, young men of the middling classes, increasingly disproportionately dominated by urban, skilled, and socially mobile activists. Officers tended to be slightly older and of slightly higher social status than their men. Sinn Fein activists were older again but otherwise shared these characteristics, as did the IRA in Britain. This dependence on urban and skilled or white-collar members, the reverse of what republicans and most historians have believed, may be attributable to a combination of the greater risks and greater organizational opportunities faced by the IRA in towns. Nevertheless, the movement did attract rural and labouring members, and did to some extent transcend class and geographical boundaries. IRA units were almost never segregated along class lines, and were usually built around familial and neighbourhood networks. Also, as the revolution progressed, activists' previous social identities were superseded by a new and essentially egalitarian identity as comrades and guerrillas.