It is generally thought that leaders are important for the performance and activities of political parties. In Fianna Fáil those leaders usually hold the office of Taoiseach, allowing Fianna Fáil to set the agenda for, and react to, changes in Irish society. We investigate the extent to which the individual choice of leader might make a difference. We assess the political and policy management of Fianna Fáil’s leaders using a series of counterfactual case studies for each leader to enable us to establish how these leaders shaped the party and the country. Taking account of the traits of the specific leader while assessing the context in which they led, and using evidence from interviews, memoirs, and a wide secondary literature, we find that in some cases the leader was instrumental in bringing about a change in direction that was unlikely to have taken place under anyone else’s leadership.