Glas Gaibhnenn (Glas Gaivlen; alternate spelling: Gloss Gavlen: pronunciation guide:/glas-gav-e-lan/;) is a prized fabulous cow of bounty (fertility) that yields profuse quantities of milk, and which belonged to semi-mythological characters in Irish oral folktales.
Power listed Glas Gamhain and Bó Bhán "white cow" (associated with Boann) as among names of mythological creatures after which landscape features or bodies of water were often named, while Patricia Monaghan wrote of Glas Ghaibhleann as a goddess often associated with rivers, and that rivers were often seen as embodiments of the goddess Boann.
The folk-tale plot has been made familiar through retellings of Irish mythology, notably by Lady Gregory, but also others where we learn that Cian mounts on a quest to recover the magic cow, has a romantic encounter with Balor's daughter, fathering the child who is to become Lugh. The folktale was stitched together using additional bona fide medieval mythological writings to form a seamless retelling. Two rather different oral versions of the folktale were pieced into one composite for this purpose.
Glas Gaibhnenn reconstructed in normalised Irish: Glas Gaibhnenn. Pronunciation guide: /Glos gov-nan/. Original (phonetic) transcription: Glas Gaivlen; alternate spellings: Gloss Gavlen: (pronunciation guide:/glas-gav-e-lan/). Additional spellings: Glas Gaibhneach, Glas Gamhain, Glas Gamhnach.
O'Donovan thought the second part of the name was a corruption of the possessive case of Goibniu "the smith", as Goibniu is Old Irish, pronounced ˈɡovʲnʲu or Gaibhne (Modern Irish). But  the bovine creature's Irish name can literally be broken down to glas "green, gray, etc." and gamuin "calf, yearling", as have been indicated by the Rev. Patrick Power, church historian and writer on topographical names.
Lady Gregory's version
Lady Gregory's reworked version can be summarised as follows: Balor of the Strong Blows (or the Evil Eye) learns from his druids that he is fated to be slain by his own grandson. Consequently, he sequesters his only daughter Ethlinn (this name has been normalised to conform with mythology, but she is called Ethnea in the original folktale). Around this time, at a place called Druim na Teine ("the Ridge of the Fire") lived three brothers, Goibniu the smith, Samthainn, and Cian together with the wonderful cow Glas Gaibhnenn. But one day when Cian comes to Goibniu's forge to have his sword wrought, leaving the other brother Samthain in charge of the cow, Balor comes along to trick Samthain into abandoning his guard, and steals away the cow back to his own island across the strait. Cian, seeks help from a druidess (and member of the Tuatha De Danaan) named "Birog of the Mountain" who informs him that the cow could never be recovered while Balor was alive. With a blast of wind she conveys Cian to Balor's tower, and penetrating the prison, allows Cian opportunity for a tryst with Balor's daughter. In the retelling, the focus switches now to the fate of the child Lugh who is born between them, so the eventual fate of the cow remains untold.
Lady Gregory makes Balor's abode to be a Glass Tower, possibly after Arbois de Jubainville.
Larminie's collected folktale
William Larminie's collected version of the folk-tale, entitled "The Gloss Gavlen" was published later than the other example, but is discussed first since it retains the name Kian for the protagonist. It has two parts, and begins with a carpenter named Gobaun Seer (Irish: saer "craftsman"; see Gobán Saor) hired to build a fine castle for Balar Beimann to boast. To prevent other lords from hiring the carpenter to build another castle to outdo his, Balar plots the carpenter's death. Gobaun survives thanks to the warnings from Balar's daughter, and now proclaims he cannot perfect his work without his three specially named tools, which he makes Balor's son fetch from his home. Upon receiving this errand-bearer, the carpenter's wife deduces the situation, and slams shut Balor's son inside the tool-chest, and with the boy as hostage demands from Balor due wages and her husband's safe return. Upon leaving, the carpenter recommends the smith Gavidjeen Go to do the ironworks for the castle, and tells Gavidjeen Go to refuse all rewards except "the Gloss", the cow which can fill twenty barrels. Balar obliged, but played the wily trick of not giving him the special "byre rope," without which the cow would stray off. The smith therefore now owned the cow but was at constant risk it may stray off, compelling him to hire champions on a daily basis to escort the cow safely back and forth from pasture, offering the forging of a sword in payment for any takers who would accept the task.
In this latter half of the tale, "Kian son of Contje" takes the offer to obtain his sword, but by carelessness allows the cow to wander off. He must now submit his head on the anvil block to have it chopped off, but requests three days of amnesty, and goes off to recover the cow. At the shore, he finds waiting "Mananaun son of Lir" in a coracle, ready to ferry him off to the whereabouts of the cow, in exchange for half of whatever Kian profits from the quest. In the land of cold, where meat is eaten raw, Kian is hired as cook, storyteller, and fireman (fire-stoker?). Thanks to Mananaun's lockpicking magic, Kian is able to frequent the chambers of Balor's daughter. When the girl bears him a son, Kian begs leave from Balor's service, and taking the infant and the byre rope, boards Mananaun's coracle. Balar discovers the situation and raises great waves and flames at sea, but Mananaun counteracts these with his greater magical prowess. Mananaun for his help obtains the child with Kian's blessing, and fosters him under the name of Dal Dauna. (This is explained as a corruption of Ildana[ch], the usual nickname for Lugh). This child one day happens upon the sight of Balar sailing past in his fleet, and tosses a dart in his pocket at Balar, thus killing him.
O'Donovan's collected folktale
In an independently collected cognate tale, Gavida, Mac Samhthiann or Mac Samthainn, and Mac Kineely (Irish: Mac Cinnḟaelaiḋ (Mac Cinnfhaelaidh)) are three brothers living on the coast of Donegal, and across the strait on Tory Island (an island named after the Tir Connell—presumably the Tower of Conand; cf. "Glass Tower" above.) lived Balor, who had one eye in front of his head, and another in the back, with the ability to petrify on-gazers. Among the three brethren Gavida was the smith, and his forge was at the Druim na Teine ("ridge of the fire").
Mac Kineely (who corresponds to Cian) was a lord of some districts and owned the coveted cow, the Glas Gaivlen, which produced milk aplenty. Balor receives prophesy from his druids that he was destined to fall by the hands of his grandson. So he locks away his daughter Ethnea in a tower built upon an inaccessible and towering rockscape called the Tor-More. Balor landed ashore to steal the cow. Mac Kineely had business with the smith, and, out of his usual habit, had entrusted the halter of the cow to his other brother. Balor then came up to this brother (Mac Samthainn) and whispered him a lie that the other two were secretly colluding to use up all his steel to build Mac Kineely's sword, and to make his out of iron," tricking him into rushing off to investigate. By the time the brothers realised, Balor had already rowed halfway down the strait with the cow on his boat. Mac Kineely had a leanan-sidhe (familiar sprite) by the name of "Biroge of the Mountain," and she would assist him in trying to vanquish Balor to recover the cow. This banshee was only able to sow the seeds of Balor's destruction, and it is not clear if the cow was ever recovered. On the wings of a storm she brought Mac Kineeley, dressed in woman's guise, into the tower where Balor's daughter lived trapped, attended by twelve matrons. Mac Kineely and the maiden fell in love, and she bore him three sons. Balor discovered this and slew Mac Kineely by a certain rock, whose red stains were still visible in the days when the folk tale was recited.
Of the three infants, just one managed to survive, and adopted by the smith Gavida, was raised as his apprentice. One day, Balor appeared on the forge, and ordered some spears to be made when only the apprentice was at the workshop. Balor let slip the fact that he had killed Mac Kineely, not realising the apprentice was the bereaved son (not named, but presumably the equivalent of Lugh). The apprentice, pretending to slave away at the forge, awaited his chance and "taking a glowing rod from the furnace, thrust it through the Basilisk eye of Balor," thus exacting his revenge.
Local geographical legend and Fenian Cycle
The remains of a dolmen in Shallee, Co. Clare is called the Leaba-na-glaise or the "Bed of the Cerulean Cow" (i.e., bed of Glas the green cow), and is alleged to be the property of a mythical smith, either Mac Kineely (same name as the hero of the prior tale), or Lon Mac Liomhtha (apparently the smith who forged the sword Mac an Luin). In the same county lies Slieve-n-glaise and one dolmen in particular erected on its slope was called Carrick-na-glaise, reputedly the abode of Lon mac Liomhtha the smith.
O'Donovan has gathered further Fenian lore, according to which, Lon the smith who took up residence here was said to be a member of the Tuatha Dé Danann. He had an extra pectoral arm for holding his tong ("Two of the hands were in the usual position, and the third, with which he turned the iron of the anvil, while he hammered with the other two, grew from the middle of his breast.") and one leg to hop on (or rather take huge leaps and bounds upon). He was for many years nourished by the cow Glas Gaibhneach which he stole from Spain, and the cow was pastured on the mountain of Sliabh-na-Glaise, not far from the forge, for no other place in Ireland was fertile enough. "This cow would fill with her milk any vessel,.. at one milking". Two women wagering on whether a vessel could be found to outsize her capacity, and when a sieve was produced, the cow's milk caused seven overflooding streams to pour forth. Also it was said "the hoofs of this cow were reversed", and the backward tracks always fooled the potential cattle-thieves in pursuit. Lon later visited Finn mac Cumhail and challenged the Fianna to a race. The fleet-footed Caílte mac Rónáin outran him in a race to the Leaba-na-Glaise, but Lon revealed the race was a friendly subterfuge to bring him to his forge so he can start crafting superior weapons for his band of Fianna warriors.
Onomastics of County Cork
The Rev. Patrick Power's Place Names and Antiquities of S. E. Cork (1917) describes several place names in the county popularly associated with a legendary cow, the Bó Bhán (white cow) and Glas Gaibhneach/Gamhain/Gamhnach (he gives these three spellings).[a]
According to Power, in the townland of Foaty (on Fota Island), in County Cork, was a pond known as Loch na Bó "Lake of the Cow", which was "supposed to derive its name from a legendary cow ― the Bó Bhán or the Glas Gaibhneach".
And in Ballyoran townland (near Fermoy) is a "Gownach Well" i.e., the well of "Gamhnach = a yearling heifer; the eponymous bovine may be the legendary Galas. G. The well: "Part of the River Ilen to the west of Skibbereen is called "Gownach," 
- O'Donovan 1856, p. 18n (specimen of oral folk tale), "a cow called Glas Gaivlen [rectè; Gaibhnenn]
- Larminie 1893, pp. 1–9(collected tale)
- Heaney 1994, p. 246
- Mackillop 1998, Dict. Celt. Myth., "Glas Ghaibhleann.. Celebrated magical cow, white with green spots, whose inexahustible supply of milk signalled prosperity"
- Rhys 1893
- Monaghan, Patricia (2010) , "Glas Ghaibhleann", Encyclopedia of Goddesses and Heroines 2, Santa Barbara: Greenwood, ISBN 9780313349935
- Gregory 1905, p. 19 (retelling)
- Heaney 1994, pp. 4–8
- Gregory 1905, p. 472
- Power, Patrick (1917). Place-names and antiquities of S.E. Cork, Ireland (Internet Archive). Dublin: Hodges. pp. 199, 205, 216.
- Larminie 1893, p. 251
- "Gloss Gavlen" means simply the Grey (cow) of the Smith, gavlen being properly gavnen―(gaibhnenn) according to O'Donovan.
- Squire 1904, pp. 447- pronounced /'sąv-ïñ/
- Arbois de Jubainville, Henry (1903), The Irish Mythological Cycle and Celtic Mythology (google), Dublin: Hodges, Figgs & Co., p. 67
- Squire & 1904 p-237, "Dul-Dana.. This name, meaning, 'Blind-Stubborn', is certainly a curious corruption of the original Ioldanach (pronounced Ildana)"
- Borlase 1897, p. 883 (local lore collected by John O'Donovan in his Ordnance Survey Letters)
- Borlase 1897,"Lon Mac Liomhtha was reported to have lived on this mountain in a cave. He was represented as a dwarf, and as the first who ever made edged weapons in Ireland."
- Borlase 1897, vol.3, p.884, quoting O'Donovan
- Borlase 1897 loc. cit. "He never walked after the usual manner of men.. but bounded from his pedesal by the elastic power of his waist and ham"
- Borlase 1897 describes his sourceas O'Donovan, O.S.L. [Ordnance Survey Letters] p.68. The letters were recently published as O'Donovan 1997.
- Power 1917, p. 205
- Power 1917, p. 199
- Power 1917, p. 216
- Mackillop, James (1998), Dictionary of Celtic Mythology, New York: Oxford University Press
- primary sources
- "Glas Gaivlen" (oral) (provisional title), told by Shane O'Dugan, Tory Island, 1835.
- O'Donovan, John (1856), Annála Ríoghachta Éireann: Annals of the Kingdom of Ireland by the Four Masters (google) 1, Dublin: Hodges, Smith, and Co., pp. 18–21
- "The Gloss Gavlen" (oral), told by John McGinty, Achill Island.
- Larminie, William (1893), West Irish Folk-tales and Romances (Internet Archive) 1, London: Elliot Stock, pp. 1–9
- "The Cow," lore gathered by O'Donovan, from John Reagh O'Cahane, tailor, of Corofin et al. (English summary with sporadic Gaelic)
- Borlase, William Copeland (1897), The Dolmens of Ireland (google) 3, London: Chapman&Hall, pp. 883–
- O'Donovan, John; O'Curry, Eugene (1997), The Antiquities of County Clare: Ordnance Survey Letters 1839 (snippet), Ennis: Clasp Press, pp. 21–, ISBN 9781900545037
- secondary sources (includes retellings)
- Lady Gregory, GaFM (first edition:1903)
- Gregory, Lady Isabella Augusta (1905), Gods and fighting men: the story of Tuatha de Danann and of the Fianna of Ireland (google), London: John Murray
- Heaney, Marie (1994), Over Nine Waves, a book of Irish legends, London: Faber and Faber, ISBN 0-571-14231-1, pp4–8, 246
- Squire, Charles (1904), Celtic Myth and Legend: Poetry & Romance (google), London: Gresham Publishing Company, pp. 233–7, ISBN 9781440042539
- Rhys, John (1886), Hibbert Lectures on the Origin and Growth of Religion as illustrated by Celtic Heathendom (google), London/Edinburgh: Williams & Norgate, pp. 314–