The last time I saw Randal, Viscount Dunluce, the 35-year-old bachelor was celebrating his purchase of a miniature at auction the previous day for half of its £1,500 high estimate. It was of his great-great-great grandmother Anne Catherine, Countess of Antrim, who, in the 1820s, had made some of the more fashionable architectural additions to his home.

“I was quite pleased with that purchase. And it came with a plait of her hair. I like to buy things that pertain to this house,” Randal said, with a wave around the drawing room of Glenarm Castle, his ancestral home in the northernmost reaches of Northern Ireland.

Five years later, when I check in with the young collector, who will one day inherit his father’s title, Earl of Antrim, I see he’s still at it, buying, selling and refining a collection deeply steeped in his family’s 300-plus-year history. Today the erstwhile bachelor is married, to wife, Aurora, with a wee heir of his own, the Honorable Alexander, a toddler who will grow up in a far different Glenarm than did his father.

“The house was quite neglected,” says Randal of the condition a decade ago, when his father passed him the keys. “When I was little all of us lived here—four generations under one roof—with my great-grandmother’s sitting room in the drawing room. The roof leaked and there had been fire damage over the years.”

But Randal has always felt affection for his Irish home and dreamed of restoring it, an ambition he’s realizing today as he commutes every fortnight from London, where he works as an investment manager. The Irish lord, whose family name is McDonnell, descends from Scotland’s Clan Donald, of which his father is a chief. Since the 16th century, they’ve had a presence in this corner of Northern Ireland.

For a collector whose family includes sculptor Angela Sykes (his grandmother), painter Hector McDonnell (his uncle), artist Flora McDonnell (his sister) and the head of the conservation department at Tate Britain, Alexander, Earl of Antrim (his father), restoring the art and antiques of Glenarm Castle is as much a priority as a new roof.

“The condition of the pictures and furniture was very poor, so I rolled out a conservation program,” he says. “My father has been cleaning the pictures for me, and once a year a furniture restorer from Belfast arrives. I have a budget, and we do what it allows. But it’s made a huge difference.”

The limited budget has forced collecting decisions at Glenarm. Randal took a hard look at the castle’s inventory and decided to focus on acquiring Irish and English antiques and paintings and, works of local interest, and on reclaiming those of Glenarm’s treasures that had been sold off over the years. To finance new acquisitions, he consigned at auction pieces that he felt didn’t fit the character of the house. “I sold a couple of vulgar Louis XV secretaries my grandfather bought back in the 1950s,” Randal explains. “I thought they didn’t look right in the house. I can do that with a clear conscience. It’s all about making the place more coherent.”