He is currently Professor of Medieval History, School of History, University College Dublin since 2004. He is also a noted academic writer on science fiction, and was the editor of Foundation - The International Review of Science Fiction from 1986 - 2001. The Cambridge Companion to Science Fiction, which he edited with Farah Mendlesohn, won the 2005 Hugo Award for Best Related Book. He has researched Late Roman and early medieval history; the history of the barbarians, particularly of the Franks; the writings of Gregory of Tours; the history of science fiction, fantasy, and utopian literature.