[1] He undertook to learn Irish, in Ballyferriter, County Kerry, at the suggestion of Carl Marstrander.
[2] In 1920, Osborn Bergin wrote a poem ('Do Ridire Mhac Giolla Chríost') to Christiansen urging him to return to Ireland.
In his The Migratory Legends (1958) he proposed a type catalogue for the classification of "migratory legends" (a calque of German Wanderlegenden, i.e. folk-tales transmitted via trans-cultural diffusion), by motif, exemplified with examples from Norwegian folklore.
He retired in 1956 and spent a semester at Indiana University Bloomington at the invitation of Stith Thompson, and 1957/9 returned to Dublin, working with Seán Ó Súilleabháin (1903–1996).
In 1958, he became chairman of the Commission International des Arts et Traditions Populaires of UNESCO.