Most of his research has been centred on the Irish Republican movement and particularly the history of the Irish Republican Army. His first book, based on his doctoral thesis, concerned the history of post Irish Civil War Republican politics and was titled Radicals and the Republic, Socialist Republicanism in the Irish Free State (1994). His next work was a biography of 1920s IRA veteran Ernie O'Malley, entitled Ernie O'Malley: IRA Intellectual which was published in 1998.
Since then, he has written Armed Struggle: The History of the IRA (2003). This book, predominantly a history of the modern Provisional IRA, won the politics book of the year award from the Political Studies Association and was shortlisted for the Christopher Ewart-Biggs Memorial Prize. After this, he wrote a broader history of Irish nationalism, Irish Freedom, The History of Nationalism in Ireland (2006), which won the Ewart-Biggs Memorial Prize in 2007.[3]
He has also co-edited two volumes: The State: Historical and Political Dimensions (1999, with Charles Townshend); and Rethinking British Decline (1999, with Michael Kenny).
He is a frequent media commentator on terrorism and Irish politics and history, including work for the BBC, NPR, The Times Literary Supplement, Newsweek, and the Financial Times.[4]
In 2009, he published a study of political violence, titled Terrorism: How to Respond.[5]
In 2016, he contributed to the documentary film Bobby Sands: 66 Days. In 2017, he delivered the Gifford Lectures on Nationalism, Terrorism and Religion at the University of Edinburgh.[6]