The Japanese animation film "Karigurashi no Arrietty", released in July 2010, is based on The Borrowers, written and published in 1952 by the British writer Mary Norton (1903-92). This paper, tracing Mary Norton's ancestry in Ireland and her experience as a writer in the same country, verifies the "Celtic Irishness" she carried in her person. First, as one of the specific characteristics of "Celtic Irishness", this paper explores the longing for the unseen and hidden things inherent in the Celts through Norton's and other Irish writer's presentations of the image of a horseman passing by in the flowing and changing world. Second, judging from the historical point of view as well as Norton's, the human beings living on the floor, and the dwarves under it, represent the rulers and the ruled, respectively. The rulers are the English and the Romans