[2] Leask was appointed the first Inspector of National Monuments in 1923, just after the foundation of the Irish Free State, and served in the position until his retirement in September 1949.

For a number of years Leask held practically sole responsibility for all elements relating to the access, assessment and conservation of the most important Irish historical and archaeological sites.

[1] He continued to study the stone built architecture of early Christian and medieval Ireland after his retirement in 1949, while taking up a post as professor of archaeology in the National College of Art and Design for a short time.

[1][2] In 1930, he was elected as a member of the Royal Irish Academy, and served on its committee for "polite literature" and antiquities as secretary for over 30 years.

He married Ada Longfield, a fellow historian and antiquary, on 31 January 1940 in St Bartholomew's church, Clyde Road, Dublin.