Slieve Gullion in south County Armagh is one of the most mythologically dense mountains in Northern Ireland: a ring of hills surrounds a volcanic plug where a passage tomb sits beside a small, dark summit lake. That lake — Loch Caillighe Birra, the Hag's Lake — is the setting for the legend known as the Hunt of Slieve Gullion (Feis Tighe Conáin in Irish). Two sisters, Áine and Milucra, both desire the hero Fionn mac Cumhaill. Knowing that Áine has vowed never to marry a grey-haired man, the jealous Milucra secretly enchants the summit lake so that any man who swims in its water will emerge ancient and white.
The story
Milucra presents herself to Fionn at the lakeside in the guise of a weeping woman, lamenting that her golden ring has fallen into the water. Fionn, ever chivalrous, dives in to retrieve it. He emerges — ring in hand — as a bent old man with white hair and withered limbs. His warriors, the Fianna, surround Milucra and force her to restore him from the cornucopia she carries; she produces a restorative drink that returns his youth and strength, but his hair never reverts to its original dark colour. This, the legend says, is the origin of his name: Fionn, meaning 'white' or 'fair,' had been a battle-name, but after Slieve Gullion it became permanent.
In several versions Milucra is identified as an aspect of the Cailleach Bhéara, the ancient hag of winter who haunts both Irish and Scottish mountain lore. The identification is geographically grounded: the passage tomb on the summit is already known as Calliagh Birra's House, and the ring of hills encircling the mountain is explicitly understood in tradition as the Cailleach's domain. Voices from the Dawn, which surveys Ireland's mythological heritage sites with academic rigour, provides detailed treatment of both the archaeological and mythological dimensions of Slieve Gullion. The summit lake retains a reputation for cold and strangeness, and wild-swimming accounts note that the water is unusually dark and frigid even in high summer.