A branch of the Eustace family - one of the most prominent 'Old English' Norman families in Kildare - held Castlemartin from the 13th century, intermarrying with the Talbots of Malahide and the Plunketts of Dunsany. In 1591 the castle burned down, according to a contemporary account, 'by the means of lewd servants.' The property reverted to the Eustaces and became the favourite home of Sir Maurice Eustace, Lord Chancellor of Ireland after the 1660 Restoration. He died childless in 1665 and the estate passed to his nephew. The current Castlemartin House was built in 1720 by a Dublin banker and MP named Francis Harrison, using stone from earlier buildings on the site. About 1730 it was sold to Thomas Carter MP, father of Captain Henry Boyle Carter. During the 1798 rebellion, British forces under Ralph Dundas commandeered the house as their Kildare headquarters; the interior was badly damaged.

By the mid-20th century Castlemartin had been declining for decades. In 1959, according to a 2018 Irish Times article, the estate was bought by Otto Skorzeny - the former Waffen-SS commando best known for leading the daring 1943 glider rescue of Mussolini from his mountain prison at Gran Sasso. Skorzeny's purchase of an Irish country estate caused political controversy at the time; he never lived there permanently and was eventually denied residency. The episode is one of the stranger postscripts to the Second World War: an estate associated with Sir Maurice Eustace and the 1798 rebellion briefly owned by a man who had been one of Hitler's favourite operators. The house deteriorated through the 1960s. In 1967 the elderly Sheelagh Blacker, the widow of Lt Colonel Frederick Blacker, left it to her great-nephew Grey Gowrie, who sold it to Dr Anthony O'Reilly in 1972.

At the point of sale in 1972, the conventional wisdom was that the house was probably only worth demolishing. Tony O'Reilly, then on his way to becoming one of Ireland's most prominent businessmen as head of the Independent News & Media group and chief executive of H J Heinz, spent millions instead on repairs and improvements. He restored the Georgian house and its outbuildings, the estate church of St Mary's (founded around 1200 and ruined for centuries), the icehouse, and the wrought-iron Grand Gate that opens onto Newbridge Road. He chose a standard teal colour for key features across the estate, which became something of a Castlemartin signature. The Castlemartin Charolais herd, founded in 1976, today comprises 325 cattle including three stock bulls and 125 breeding cows. For decades Castlemartin was the principal Irish home of the O'Reilly family. After O'Reilly became tax-resident in the Bahamas, it was a second home he shared with his wife Chryss Goulandris - a shipping heiress and prominent breeder herself - and the place where the family gathered every Christmas.

Financial pressures - O'Reilly's media empire collapsed in the wake of the 2008 financial crisis - meant Castlemartin went on the international market in late 2014. John Malone, the American Liberty Media chairman and one of the largest private landowners in the United States, bought it in 2015 for what was that year's most expensive Irish residential sale. The house remains largely closed to the public. So does the chapel of St Mary's, despite promises made at the time of its restoration that public access would be maintained. From the public roads, neither the house nor the church can be seen. Castlemartin has spent eight centuries acquiring privacy and seems determined to keep it.