Her parents were Major Alfred Percival Longfield (1862–1916) and Constance Ada (née Sanders) (died 1967) of Edinburgh.
While attending Trinity College Dublin, she won prizes each year, and graduated in 1921 with a BA and LLB.
Having briefly worked as a teacher, she took up a position in the National Museum of Ireland in 1932 in the art and industrial section.
[4] She continued with her own research, writing books and articles on Irish lace, delftware, tombstones, embossed pictures, wallpaper, wall-paintings, and textiles.
She worked with the Irish Manuscripts Commission, resulting in the Shapland Carew papers in 1946 and the Fitzwilliam accounts 1560–65 in 1960.