O'Malley's extensive notes, compiled while he was an active IRA officer, are one of the best primary historical sources for the revolutionary period in Ireland, 1919–23, from the republican perspective. In the 1930s and 1940s he also toured Ireland interviewing veterans of the republican struggle. His papers are now housed in the University College Dublin archives, to whom they were donated by O'Malley's son, Cormac, in 1974. Cormac O'Malley retains the bulk of the remainder of his father's personal papers, poetry, and some manuscripts in his New York residence.