Complex sentence: The film was considered lost for many years until in 1991 a single surviving nitrate print was located in the US Library of Congress by the Irish Film Institute's Irish Film Archive who had the film transferred to safety stock and restored.
Split into simpler sentences: The film was considered lost for many years until in 1991 a single surviving nitrate print was located by the Irish Film Institute in the United States'Library of Congress. The institute's archive had the film transferred to safety stock and restored.