Long before any rhododendron grew here, this promontory was the seat of the Mac Finin Dubh O'Sullivan family, a branch of the O'Sullivan Beare, who held the land from around 1320. After the Cromwellian conquest of Ireland in the 1650s, the property was confiscated and granted in 1657 to Sir William Petty, Oliver Cromwell's physician, as a reward. Petty was a remarkable polymath who carried out the Down Survey, mapping confiscated Irish land in unprecedented detail, and his descendants - through his daughter Anne's marriage in 1692 to Thomas Fitzmaurice, First Earl of Kerry - became the Lansdownes. One of them, the Second Earl of Shelburne, served as Prime Minister of Great Britain and was made Marquess of Lansdowne in 1784. The O'Sullivans stayed on, demoted from chieftains to tenants, a pattern repeated across Ireland.

In 1870, the Fifth Marquess of Lansdowne, then 24 years old and newly come into the estate, looked at the bare rock and scrub oak around Derreen House and decided to plant a garden. By tradition, he employed 40 people to clear, drain, and plant 400 acres of land. From his postings abroad, he sent home shipments of seeds and seedlings. From the Veitch Nursery in England, then the leading purveyor of exotic species, he bought rare specimens. He subscribed to Himalayan plant-hunting expeditions, those Victorian collecting trips that combined botany with imperial reach. The result, growing into maturity over three or four decades, was the place visitors see today: 60 acres of woodland threaded with nearly 12 kilometres of paths, the moist Atlantic air carrying enough warmth from the Gulf Stream to keep frost rare and the moss perennially wet.

In 1903, King Edward VII and Queen Alexandra visited Derreen House. They planted two commemorative bamboos. Both are still alive, somewhere in the garden, though after more than a century of vigorous spreading they have lost any sense of being two distinct things. The garden is known particularly for its rhododendrons - especially the Himalayan Rhododendron arboreum - and for those tree ferns, which in this climate grow tall and thicken into improbable forests. Paths wind through outcrops of bare rock, with sudden views opening onto Kilmakilloge Harbour and the Caha Mountains beyond. MacGillycuddy's Reeks rise distantly to the north. The whole effect is of a place that has forgotten which hemisphere it belongs in.

The Lansdownes were absentee landlords, summer residents only. They visited for three months a year, except during long absences for imperial service and during the Irish War of Independence and Civil War, when it was not safe for them to come. In 1922, during the Civil War, Derreen House was plundered and burned. The Fifth Marquess rebuilt it in 1924, with some financial help from the new Irish Free State - a notable gesture for a state then very much defining itself against the Anglo-Irish gentry. The seventh Marquess was killed in action in August 1944, in Italy. The estate passed sideways through the family, eventually to his sister, Lady Katherine Petty-Fitzmaurice, and from her to her grandson, Charlie Bigham, who owns Derreen today. The garden remains open every day of the year.