Regan-Lefebvre's earlier work investigated the experience of the Irish in the British Empire, and particularly the intersections of nationalism, imperialism, and colonialism.  Her first book was a biography of the Irish Quaker nationalist, and president of the 1894 Indian National Congress, Alfred Webb.  Cosmopolitan Nationalism in the Victorian Empire revealed a strain of Irish nationalism that was globally-minded, civic, and committed to imperial reform.  She also edited and published the memoirs of J. F. X. O'Brien, revealing his radical engagements in France, New Orleans, and Nicaragua before his entry into Fenian politics. 

Regan-Lefebvre teaches widely across British and world history and enjoys developing new classes to stimulate students' interest in the past and to build their skills as historians and communicators.  Her classes include an introductory British history survey, seminars on British cultural history, wine history, Irish history, and historical networks, and an experiential course in parliamentary debate.  She has been particularly proud to supervise a number of senior thesis writers in undertaking archival research in the UK.  Her former Trinity research students have gone on to pursue graduate degrees in history at Cambridge, Yale, and Carnegie Mellon.