There has been a castle at Glenarm since the 13th century. The first builders were the Bissets, a family who had acquired the lands between Larne and Ballycastle from Hugh de Lacy, Earl of Ulster. John Bisset made Glenarm his capital, and by 1260 the village had a castle at its centre, complete with kitchen garden, orchard, mill, and the woods and meadows of a working medieval estate. The old village courthouse still incorporates fragments of those original walls. The discovery in the 1970s of an immured skeleton built into the masonry tells a story the documents do not. Whoever was placed inside that wall - sacrifice, punishment, or unfortunate workman - went into the stone of Glenarm Castle and stayed there for seven hundred years.

In 1495, the Bisset chieftain of Glenarm was so renowned in the country that a rival named Con O'Donnell of Tirconnell decided he simply had to have what the Bisset had. According to the chroniclers, O'Donnell had been told that MacEoin of the Glens - as the Bisset chief was known in Irish - had the finest wife, finest steed, and finest hound in the neighbourhood. O'Donnell sent messengers to ask for the horse and was refused. So he marched east through every difficulty of the passage and arrived at MacEoin's house at night without warning. He captured MacEoin, and made himself master of the wife, the steed, and the hound. The last MacEoin Bisset was killed fighting the O'Donnells in 1522. Their lands were swallowed up by the MacDonnells, the same Scottish-Irish family whose dominance of the Glens had been shattered at Glentaisie in 1565 - and who had then patiently rebuilt themselves into the new lords of Antrim.

The current castle was built in 1636 by Sir Randal MacDonnell, 1st Earl of Antrim, the man who had successfully turned MacDonnell power into a hereditary English title under James I. He chose the same site the Bissets had used and laid out the new house at the head of the village, with a long avenue running back from the gate. Generations of remodelling have given it the gothic-revival outline visitors see today, but the bones of Sir Randal's 17th-century house remain inside the modern walls. The current owner, Randal McDonnell, 10th Earl of Antrim, lives there with his family. The McDonnells have been in Glenarm village for nearly six hundred years and in the present castle for almost four hundred. Few estates in either Ireland or Britain can claim that kind of dynastic continuity.

Since the 1990s, Glenarm has been opening itself up. The walled garden is open to the public between May and September, with one of the finest collections of fruit trees, herbaceous borders, and Victorian glasshouses in Ireland. The grounds host an annual Highland Games every July, a reminder that the McDonnells came originally from the Scottish Isles and have never lost the cultural connection. The Dalriada Festival, named for the ancient Gaelic kingdom that once united the Glens of Antrim with the Scottish Highlands, runs at the castle every summer. In 2012 the grounds began hosting outdoor concerts - Ronan Keating, Sharon Corr, The Priests, Duke Special - and Summer Madness, Ireland's biggest Christian Festival, made Glenarm its new home after years at the Kings Hall in Belfast.

Like much of the Antrim coast, Glenarm Castle has more recently become a film set. It was used as a major location in Five Minutes of Heaven, the 2009 drama starring Liam Neeson and James Nesbitt about reconciliation in post-Troubles Northern Ireland. And the grounds of the castle estate served as the location of the Ashford Meadows tourney in the first season of A Knight of the Seven Kingdoms, the HBO Game of Thrones spinoff. Where the Bissets once feasted, where Sir Randal once received his earldom, where the 9th Earl played host to wartime guests during World War II, modern television crews now film knights at the joust. The cast and the cameras leave. The McDonnells stay.