At the summit is a small lake and two ancient burial cairns, one of which is the highest surviving passage grave in Ireland.

Slieve Gullion appears in Irish mythology, where it is associated with the Cailleach and the heroes Fionn mac Cumhaill and Cú Chulainn.

The geological formation was the first ring dike to be mapped,[5] although its significance was not understood until similar structures had been described from Scotland.

Some bits of worked flint and a barbed-end arrowhead were also found, "the meager remnants that survived the centuries of tomb raiding".

[21][22] On the eastern side of the mountain is Slieve Gullion Forest Park, which includes a visitors' centre, café, playground, and the Giant's Lair Story Trail.

In the tale known as The Hunt of Slieve Gullion, Áine and her sister Milucra both seek after the legendary hero Fionn mac Cumhaill (Finn McCool).

Knowing that Áine vowed never to marry a man with grey hair, Milucra secretly puts a spell on the lake atop Slieve Gullion, so that anyone who swam in it would become elderly.

She tricks Fionn by asking him to fetch her golden ring from the lake, and he emerges as an old man with grey-white hair.

In some versions of the tale, Milucra is revealed to be the Cailleach Bhéara (Calliagh Birra), an ancient goddess.

[26] Slieve Gullion is said to be where the legendary hero Cú Chulainn (Cuhullin) received his name and where he spent his childhood as Sétanta.

Culann invites Conchobhar mac Neasa, king of Ulster, to a feast at his house on the slopes of Slieve Gullion.

Conchobhar goes ahead, but he forgets about Sétanta, and Culann lets loose his ferocious hound to guard his house.

When Sétanta arrives, the hound attacks him, but he kills it; in one version by smashing it against a standing stone, in another by driving a sliotar (hurling ball) down its throat with his hurley.

In the Táin Bó Cuailnge, the nearby Gap of the North is where Cú Chulainn single-handedly fends-off the army of queen Méabh.