The Irish name is Sliabh Cuilinn - Culann's mountain - and it tells the story of how the Ulster hero Cu Chulainn got his name. According to the Ulster Cycle, the smith Culann lived on the slopes of Slieve Gullion. He invited King Conchobhar mac Neasa to a feast. On the way the king passed a hurling field and was so impressed by a boy named Setanta that he asked him to come along after the game. The king forgot to mention this to his host. When Setanta arrived, Culann's enormous hound attacked him, and the boy killed it - in one version by smashing it against a standing stone, in another by driving a sliotar down its throat. Culann was bereft. Setanta promised to rear him a replacement pup and to guard the house himself until the new dog grew. The druid Cathbhadh proclaimed his name: Cu Chulainn, the Hound of Culann. Nearby, at the Gap of the North, the grown hero would single-handedly hold off Queen Medb's army in the Tain Bo Cuailnge.

Slieve Gullion is the eroded heart of a volcano that erupted roughly 60 to 66 million years ago, during the Paleocene, as the Atlantic Ocean was opening and Greenland was tearing away from Europe. The mountain itself is built of layered igneous rocks - basic and acidic magmas that interacted in unusual ways, producing rock relationships that have fuelled international geological debate since the 1950s. The Ring of Gullion, the circular formation of hills surrounding the central peak, was the first ring dyke ever mapped on Earth. Glaciers from the last Ice Age scoured the area, leaving the mountain with a classic crag-and-tail profile: the hard volcanic core resisted the ice, while glacial debris piled up in the lee to form a long southern ridge that ends at Drumintee. About 612 hectares of dry heath on the mountain are now designated a Special Area of Conservation.

Two ancient cairns stand on the summit, one on each side of the small lough. The southern one is the prize: a passage tomb 30 metres across and 5 metres high, with a chamber 3.6 metres wide and a corbelled stone roof reaching 4.3 metres. It is the highest surviving passage tomb in Ireland. Radiocarbon dating puts its construction at roughly 3500 to 2900 BCE. The entrance is aligned with the setting sun on the winter solstice, so that on the shortest day of the year light pours into the chamber and briefly illuminates the inner basin stones. Excavations in 1961 found fragments of human bone, worked flint, and a barbed-end arrowhead - what one archaeologist called "the meager remnants that survived the centuries of tomb raiding." Locals know the tomb as Cailleach Beara's House. The lough is Cailleach Beara's Lough. A rock feature lower down, on a hillock called Spellick, is the Cailleach Beara's Chair, where people used to take turns sitting at Lughnasadh.

Around 1680, the wave of anti-Catholic hysteria touched off in England by Titus Oates and his fabricated Popish Plot reached the slopes of Slieve Gullion. A priest named Father Mac Aidghalle was celebrating mass at a rough stone altar on the mountain - one of the open-air mass rocks Irish Catholics used when their faith was illegal - when a company of redcoats led by a priest-hunter named Turner arrived and killed him as he stood at the altar. Local oral tradition says that Redmond O'Hanlon, the outlawed Chief of Clan O'Hanlon and the most notorious rapparee in south Ulster, avenged the priest. The act sealed O'Hanlon's own fate; he was killed in 1681. The mass rock still stands on the mountainside. So does the memory.

Roughly 20,000 people climb Slieve Gullion every year. A road winds halfway up the western side to a small car park, and a footpath leads from there to the summit cairn. The eastern slope holds Slieve Gullion Forest Park, with a visitors' centre, a cafe, a playground, and the Giant's Lair Story Trail - aimed squarely at children but full of references to the older Fionn legend. On a clear day the view from the top reaches County Antrim to the north, Dublin Bay and County Wicklow to the south. American GIs training here during the Second World War disturbed the cairns. In recent years volunteers have helped repair them under archaeologist supervision. Folklore is firm: do not disturb such tombs. The penalty is a curse. The mountain has been keeping its secrets for five thousand years and shows every sign of keeping them for five thousand more.