Dr Campbell’s research initially explored intellectual life in early modern Ireland, published as Renaissance Humanism and Ethnicity before Race, and his chapter on early modern intellectual history for the new Cambridge History of Ireland.

While he has previously published on both Catholic and Protestant political thought, he is currently writing a history of Franciscan political thought in seventeenth-century Rome. This is part of a wider research project, funded by the European Research Council, into the relationship between faith and warfare in early modern Europe.