Professor English's research focuses on the politics and history of nationalism, political violence, and terrorism, with a particular focus on Ireland and Britain.

His books include Does Terrorism Work? A History (OUP, 2016), Modern War: A Very Short Introduction (OUP, 2013), Terrorism: How to Respond (OUP, 2009), Irish Freedom: The History of Nationalism in Ireland (Pan Macmillan, 2006), Armed Struggle: The History of the IRA (Pan Macmillan, 2003), Ernie O'Malley: IRA Intellectual (OUP, 1998), and Radicals and the Republic: Socialist Republicanism in the Irish Free State 1925-1937 (OUP, 1994).