The Gap of Glan, the dramatic cleft that gives the valley its character, was supposedly created by a cow. The cow in question was Glas Ghaibhleann, the celebrated magical cow of Irish mythology, white with green spots, whose inexhaustible supply of milk signalled prosperity. MacKillop's Celtic Dictionary lists multiple spellings and even more disputed origins. Possibly she belonged to Goibniu the smith, possibly to Gaiblin, a farmer of County Cavan, possibly to Balor the Fomorian of Tory Island. According to the Glangevlin version, when the cow ran away from the blacksmith's forge, she made the gap. The forge itself can still be located in the townland of Derrynatuan. The cow's name lies inside the valley's name: Gleann Gaibhneann, Gleann Gaibhle, the spellings shifting across centuries but always referring back to the milk-giving creature whose flow of plenty made the place memorable.

Glangevlin appears in writing for the first time around 1180, in a poem called Acallam na Senorach where the warrior Cailte mac Ronain meets Saint Patrick and tells him tales of the Fianna. In one tale, Fionn mac Cumhaill is captured by High King Cormac mac Airt, and Cailte must bring two of every wild animal in Ireland to win his freedom, including 'two chough birds from Glangevlin.' The Book of Magauran contains multiple references between 1290 and 1344, in poems composed by court poets of the McGovern clan whose chiefs ruled this glen. Poem after poem celebrates Gleann Gaibhli's swift steeds, its hero who guards its borders, the wars between its rulers and rival clans. In 1390, a war broke out between Tigernan O'Rourke, king of Breifne, and Thomas O'Reilly. The Annals of Loch Ce, Connacht, Ulster, the Four Masters and Clonmacnoise all describe O'Rourke receiving word of an invasion 'whilst he was in Glenn-Gaibhle' and conducting a counter-attack that drove the invaders back from his border.

In early 1708 the most famous Irish harpist of his era, Turlough O'Carolan, was travelling from Fermanagh to Mayo when a snowstorm caught him in Glangevlin. He and his guide took shelter in what he later described as 'a miserable cabin,' where they waited several days for the snow to clear. The conditions were brutal, the food was poor, and the water was the only drink. From that experience O'Carolan composed one of his most famous airs, the Lament for Sir Ulick Burke. Another of his major compositions, The O'Rourke's Feast, was based on a poem called Plearaca na Ruarcach written by a native of Glangevlin, Hugh McGovern (Aodh Macgowran) around 1712. The remote glen that snowed O'Carolan in produced two of his lasting works, an unexpected dividend on bad weather.

In May 1836, the great Irish scholar John O'Donovan visited Glangevlin for the Ordnance Survey. He left an extraordinary record of what he found. 'After having procured a kind of a dinner at the head Inn of Swanlinbar,' he wrote, 'we directed our course southwestwards for about three miles through the Parish of Kil Naile, and then turned northwestwards to make our way into the centre of the wild valley of Glen Gavlen, a distance of 8 long Irish miles. This is the worst road and perhaps the wildest district I ever saw.' He stayed two days in a farmer's house. On the second day he walked to the townland of Derrylahan to visit 'the large spring well in which the Shannon (according to tradition) had its source.' He noted the locals 'speak the Irish very well' and recorded the tradition of St Bridget and St Leyny, including the startling detail that when Leyny told Bridget he loved her eyes, she plucked them out and gave them to him. Leyny then repented, became a saint, and built a church.

Glangevlin parish contains forty-two named townlands, each a small unit of land that has carried its name for centuries. Altnasheen, Altshallan, Bellavally Lower, Bellavally Upper, Bursan, Carnmaclean. Their names, mostly in Irish, describe slope, hill, valley, ridge, mountain pass. The Plantation of Ulster brought the lands into English hands in 1609, granted first to John Sandford of Castle Doe, then sold to Toby Caulfeild, 1st Baron Caulfeild, in 1620. By the 1663 Hearth Money Rolls there were eleven Hearth Tax payers in the place, with names like Tirlagh O Davin, Daniell McGawran, Patricke McGwire, Phelemy Oge O Dolan. By 1717 Morley Saunders was the owner. By the 1938 Duchas folklore collection, the glen had a wealth of oral tradition still being recorded by schoolchildren. The Roman Catholic parish, separated from Templeport in 1750, still serves this scattered community. And Maguire's Chair, a large stone four miles from the village, marks where the Maguire clan were once inaugurated as chiefs in medieval times. A glacier-carved glen, a magical cow, a harpist's lament, a saint's plucked eyes, a chief's inauguration stone. Few small Irish parishes carry more story per acre.