McLysaght was elected to the Royal Irish Academy in 1942 and in the same year was awarded a D.Litt. He was appointed Chief Herald of Ireland in 1943 and served in this post until 1954.
McLysaght served as Keeper of Manuscripts at the National Library of Ireland from 1948 to 1954 and was Chairman of the Irish Manuscripts Commission from 1956 to 1973.