The Milky Way in Irish mythology and folklore

The Milky Way has been known by several names and phrases in Irish mythology and folklore. Principal among its names seems to have been Bealach na Bó Finne, which means the Way of the White Cow. In this respect, it seems to have been regarded as a heavenly reflection of the River Boyne. A variant of this is Bóthar na Bó Finne, the Road of the White Cow. The goddess Bóinn's name is from Bó and Find, meaning White Cow. In the Dindshenchas, we are told that the Boyne river was formed when she approached Nechtain's Well and it overflowed, washing her along the river, mutilating her, and finally carrying her out to sea where she was drowned. There, we are told, her lapdog Dabilla was turned into the Rockabill Islands. This is undoubtedly a creation myth.

The Milky Way (Bealach na Bó Finne) over the River Boyne (Abhainn Bó Finne).

There are other names and phrases for the Milky Way. These include:

Ceann Síne - Síne is "chain", and ceann is "head" or "chief", perhaps even an "end point". (An end point would be interesting with regards to the movement of the sun, moon and planets along the ecliptic, which intersects the Milky Way at two points in the sky. Perhaps one of these points was considered the place where the heavenly cycles began and ended?)

Síog na Spéire - the streak or stripe of the sky.

Earball na Lárach Báine - the tail of the white mare. In folklore, Láir Bhán is also a phrase for the moon. The Láir Bhán is considered by some to be an ancient sovereignty goddess. One old Samhain custom in Ireland involved the procession of the Láir Bhán (White Mare) from house to house. People would blow on cows' horns and the party was headed by a person dressed in a white robe or sheet who was known as the "White Mare".(1)

Mór-Chuing Argait - A name given for the Boyne in the Dinshenchas meaning "Great Silver Yoke", which might also have been a description of the Milky Way.

Smir Find Fedlimthi - the White Marrow of Fedlimid, also from the Dindshenchas poem Boand I.

Claí Mór na Réaltaí - the Great Fence/Ditch of the Stars.

Sgríob Chlann Uisnich - Track of the children of Uisneach.

This last one is contained in a beautiful folk memory, recalled in Scotland and in Nova Scotia but relating to an Irish myth (Deidre and the Sons of Uisneach) which appears to be the recounting of an ancient creation myth about the Milky Way. In this story, the Milky Way is known as Sgríob Chlann Uisnich. 

From Deirdre and the Lay of the Children of Uisne.(2)

It is not surprising to see the Milky Way described in conjunction with the swan. The constellation we know today as Cygnus may have been important to Stone Age astronomers and it appears to fly along the heavenly river.

In the Nova Scotia version of Deirdre and the Sons of Uisneach, the origin of the Milky Way galaxy is depicted as emerging from two trees as separated by a loch, as if to complete an arch between them.This episode is placed in the well-known Ulster tale if Deirdre, whose lover, Noíse, is one of the Children of Uisneach.

... the sons of Uisneach are killed in a great, unnamed battle, after which Deidire falls into the grave with the men. The bodies of the two lovers are exhumed and reburied on either side of the burial mound. Soon a tree grows from each grave and rises until the two join. This arouses a great deal of vengeful malice in an unnamed king, who orders that the trees be cut down. Soon another pair of trees grows and joins until the king has them cut down as well. This sequence of events recurs repeatedly until the king decides to have the bodies placed on either side of a loch, a distance too great for the trees to span. Between the trees a cluster of stars gathers in a light trail, Sgríob Chlann Uisnich [track of the Children of Uisneach].(3)

Sgríób Chlann Uisnich - the track of the Children of Uisneach - over the Dowth megalithc mound.

References:
(1) Paice MacLeod, Sharon (1960), Celtic Myth & Religion, p.175.
(2) Carmichael, Alexander (1914), Deirdre and the Lay of the Children of Uisne, Hodges, Figgis & Co., p.152.
(3) MacKillop, James (2006), Myths and Legends of the Celts, Penguin Books, p.292.