Runkerry House is one of Northern Ireland’s grand country homes, a landmark Victorian-era manor overlooking the ocean near the Giant’s Causeway.

Built by Edward Macnaghten in 1885, it boasted 20 bedrooms, 5.6 hectares (14 acres) of gardens and a beautiful location by Bushfoot Strand in Portballintrae, County Antrim.

It has embodied not just grandeur but generosity because in 1950 Macnaghten’s son, Sir Malcolm Macnaghten, gifted the estate, worth £3.25m in today’s money, to Northern Ireland’s government.

But there was, it turns out, a less than philanthropic reason for such munificence. Newly declassified documents reveal Macnaghten, an MP and high court judge, wanted to keep the property out of Catholic hands.

“I could no doubt sell it at a price but I am very unwilling to do so lest it … fall into the hands of the Roman Catholic Church,” he said in a letter to the unionist-dominated government at Stormont. “The idea that the house should become a RC monastery or convent or seminary is abhorrent to me as it would have been to my father.”

The statement, first reported this week by the Belfast News Letter, casts Runkerry House in a new and unflattering light as a symbol of the brazen sectarianism of a state set up to ensure a Protestant majority: the Northern Ireland prime minister, Basil Brooke, had once urged an audience “to employ good Protestant lads and lassies only, and not Catholics”.

The disclosure also tarnishes the reputation of Macnaghten, an establishment scion who through his family was linked to social reform and leftwing activism in London. His resolve to keep Runkerry House out of Catholic control reflected a mindset common to unionist and Protestant leaders of that era, said Graham Walker, a historian at Queen’s University.

“There was an obsession about losing territory,” said Walker. “They pointed to Catholic numbers increasing over the years and so there was this notion of holding on to symbolic territory.”

Macnaghten made his offer in a letter to Sir William Scott, the head of the Northern Ireland civil service. It was dated 22 March 1950 and sent from his Campden Hill Court address in Kensington.

The former judge and Derry MP, then aged 80, said his sister’s recent death had left him in possession of Runkerry: “A large house, occupying an incomparable site with a view of Donegal in the distance, with 20 bedrooms as well as spacious attics for the staff of domestic servants and rooms in the basement for the male servants.”

He had no use for it but feared selling lest the Catholic church snap it up, as it had another prominent property in nearby Portglenone, he said. “The house is a prominent feature on the landscape. The district around it is almost wholly Protestant.” He said he would be “very pleased if our NI government would accept it as a gift from me”. The government accepted.

Macnaghten’s 69-year-old letter was among files from the 1990s recently released by the Public Record Office. Officials studied the letter in 1994 because of plans to sell the estate, which was costly to maintain, and an attempt by Macnaghten’s grandson, Sir Patrick Macnaghten, to claim it.

The letter’s sectarianism shocked Graham Thompson, who was a civil servant in the Department of Finance and Personnel. “It was so blatant,” he recalled. “It really jumped out. It was an insight into how people thought at that time. You would not see such blatant statements written down any more.”

Sir Malcolm Macnaghten, the son of a lord, educated at Eton and Cambridge, was a product of the conservative establishment. But progressive politics pervaded his home. He married a daughter of the social reformer Charles Booth and all three of their daughters became socialists.

Macnaghten’s hostility to the Catholic church reflected unionist anxiety that the church was aiding the Anti-Partition League, a nationalist movement dedicated to a united Ireland that had its own sectarian figures, said Walker.

“There was this idea that the minority wanted political power and that the minority were disloyal and wanted to destroy the state,” he said.

Macnaghten died in 1955. His grandson failed to wrest Runkerry back from the state in the 1990s: the developer Seymour Sweeney bought it and turned it into luxury apartments.