The Kelly Library Collections are particularly strong in the areas of philosophy, the history of Ireland, Celtic languages and literature, British and Canadian history, book history, English literature, and the history of the Middle Ages. The Catholic theological collection, perhaps the strongest such collection in Canada, emphasizes patristics, early and medieval church history, Thomism, the Bible (especially Canon, Johannine literature, and the history of criticism), liturgical renewal, religious education, and ecotheology.

The Kelly Library actively collaborates with the main university library in the selection, maintenance and delivery of one of the worlds most extensive collections of electronic scholarly resources.

The Archival and Manuscript Collections includes the papers of individuals and organizations collected by the Kelly Library for primary source research with particular emphasis on supporting the research and teaching mission of the College programs (Celtic studies, Christianity and Culture, Mediaeval Studies and the Book and Media Studies) and the Faculty of Theology. 

Private papers that enrich the research component of the College programs including the papers of Canadian writer Sheila Watson and Marshall McLuhan bibliographer James Feeley, Irish politician Patrick O’Neill, The New Catholic Times Inc. fonds among others. The Collection is particularly strong in the areas of Christianity, Christian spirituality and Catholicism. The archival collections that document the Christian spiritual and faith experience including the Henri J.M. Nouwen Archives and Research Collection and records related to Jean Vanier and his communities of l’Arche and Faith and Sharing.The department also houses archival collections that complement the Kelly Library Rare Book Collection including an extensive 19th century French theatre collection and a small archives of intellectural and writer G.K. Chesterton.