The Celtic Collection is the largest component of the department's rare book holdings and came to the library primarily in three major donations:
- The Ancient Order of Hibernians of Minnesota gave 500 titles on then-contemporary Irish politics and on 19th century Irish history to the library in 1917, to commemorate the 1916 Easter Rising in Ireland.
- In 1936, Sisters Mary Agnes and Mary Fidelis O'Connor, nuns of the Visitation Convent of St. Paul, donated the splendid miscellaneous rare book collection of their father, Peter O'Connor (d. 1916) to the college library. This collection of more than 2,000 titles focuses on the history (particularly the local history), church history, religion, antiquities, folk-lore, art, and music of the Celtic nations, emphasizing Ireland and Scotland.
- James P. Shannon, President of the College of St. Thomas (1956-1966), arranged for the purchase of the scholarly library of Eamonn O'Toole, formerly professor of Irish at Trinity College, Dublin, when the latter died in 1956. This collection, also of over 2,000 titles, primarily reflects O'Toole's interest in the Irish and Scots Gaelic, Welsh, Manx, Cornish, and Breton languages and national literatures.
The department also features the Lawrence O’Shaughnessy Collection of Irish Poetry which includes rare and first editions of modern Irish poetry. Prominent among these texts are collections of the Dolmen Press, the Cuala Press, and the Raven Arts Press; works by Nobel laureate Seamus Heaney and Nobel nominee Austin Clarke; and a collection of early-twentieth century nationalist poetry.All of the items in the Celtic Collection are cataloged and are entered in into LibrarySearch. Look for holdings designated by the locations OSF-Special or OSF-Special Oversize.