The castle was built by Captain John George Adair (1823-1885), a native of County Laois, and a member of the minor gentry. Adair had made his fortune by chancy land speculation in the United States, and he returned to Ireland and bought up vast tracts of land in Donegal. Adair had married in 1869, Cornelia Wadsworth Ritchie, a daughter of James S. Wadsworth, a Union general in the American Civil War. Together they set about the creation of the gardens and castle. Adair's ambition was to create an estate and castle that surpassed Balmoral, Queen Victoria's Scottish retreat. John Adair is remembered with scant affection in Donegal. On the heels of the Great Famine and emigration on a par with the Highland Clearances, John Adair evicted 224 tenants from their blackhouses on his land. This was not for financial gain, but merely to improve the aesthetic aspect from the castle. These tenant clearances are known as the "Derryveagh Evictions". The name of John George Adair as a Donegal landlord has passed into history and folklore, ballad and documentary. All have one thing in common – Adair was notoriously cruel. He purchased Glenveagh and Gartan in 1859 making an estate of .