Leask, Harold Graham (1882–1964), architectural historian and archaeologist, was born 7 November 1882 in Dublin, probably at the family home in Harold's Cross, youngest among (at least) three sons and three daughters of Robert H. Leask, Scottish-born architect and engineer, and Anna Louisa Leask (née Molloy). The family was presbyterian. Robert was apprenticed to his father (1898–1902) and then worked with a firm of Dublin ironfounders and in a Waterford drawing office before spending two years in the office of G. P. Sheridan. He then rejoined his father, but left in 1908/9 to take up employment in the office of public works as assistant surveyor. In 1923 he was appointed first inspector of national monuments in the newly founded Irish Free State, and he remained in this post until his retirement in September 1949. For years Leask was almost single-handedly responsible for organising all aspects of assessment and conservation of, and access to, Ireland's important archaeological and historical sites. He managed to have more and more of them scheduled as national monuments, during a time when there was little public interest in particular aspects of Ireland's past, and particularly limited concern for the Anglo-Norman heritage. Even less interest was manifested by the politicians who controlled financial allocations, and Leask's achievement – in a chronically under-funded and almost unstaffed department – was heroic. It is probable that at least some of the sites which Leask scheduled for preservation would not otherwise have survived official antagonism, public neglect, and individual opportunism during the first half of the twentieth century in Ireland.
During his long career, which lasted well beyond his official retirement, Leask contributed more than anyone else to the study of the architecture in stone of early Christian and medieval Ireland. His detailed knowledge of individual sites throughout the country, gained during his long service to the office of public works, gave him a unique opportunity to deal authoritatively with architectural styles and trends in Ireland as a whole. He contributed scholarly notes and articles, describing and illustrating ancient buildings and other archaeological sites, to numerous periodicals, in particular the RSAI Journal, and he wrote the standard guides to such important national monuments as Glendalough, Co. Wicklow, and the Rock of Cashel, Co. Tipperary. His most important contributions, synthesising the results of decades of painstaking research, are his Irish castles and castellated houses (1941) and the three volumes of his Irish churches and monastic buildings (1955, 1960), which, though dated in certain respects, remain essential and well-illustrated reference works. Leask was also an authoritative and respected speaker and lecturer, generously acting as a guide on numerous field excursions of the RSAI.
After his retirement, he was for a short time professor of archaeology in the National College of Art, Dublin. He was elected to membership of the RIA in 1930, and was secretary of its committee for polite literature and antiquities for over thirty years. He was president of the RSAI 1941–4 and, for his outstanding services to the society, received an honorary fellowship of the RSAI in 1950; he was awarded the honorary degrees of master of architecture by the NUI (1942), and LLD by TCD (1951).
He lived with an older sister until her death in January 1939. On 31 January 1940, in St Bartholomew's church, Clyde Road, Dublin, Leask married the historian and antiquary Ada Longfield (Ada Leask (qv)). He died 25 September 1964, survived by his wife.