古愛爾蘭語
Perhaps the most important German-speaking Celticist is the Swiss scholar Rudolf Thurneysen (1857–1940). A student of Windisch and Zimmer, Thurneysen was appointed to the chair of comparative linguistics at the University of Freiburg in 1887; he succeeded to the equivalent chair at the University of Bonn in 1913. His notability arises from his work on Old Irish. For his masterwork, "Handbuch des Altirischen" ("Handbook of Old Irish", 1909), translated into English as "A Grammar of Old Irish", he located and analysed a multitude of Old Irish manuscripts. His work is considered as the basis for all succeeding studies of Old Irish.