A native of Ireland, Killian has a master's in archaeology from the National University of Ireland, Galway and a doctorate (PhD) in archaeology from University College Dublin, Ireland. Killian is a former fellow at the University of Barcelona, where he moved to in 2011. His area of specialism is prehistoric archaeology - hunter-gatherers, and early farming (Neolithic) communities. Killian also has a broad interest in more ‘recent’ history and archaeology, from the Roman, medieval, and modern periods, in particular the evolution of urbanism and cities, and the interplay between town and country life.