Tómas Ó Cathasaigh's research and teaching deal with Old and Middle Irish language and literatures, in particular narrative texts, historiography, and mythology. He is the author of a number of articles on Irish heroic texts such as the Cath Almaine and Täin Bö Cuailnge, as well as The Heroic Biography of Cormac Mac Airt (Dublin Institute for Advanced Studies, 1977) and Coire Sois, the Cauldron of Knowledge (University of Notre Dame Press, 2014).