The Goddess at Lughnasadh

Taltiu, slow Magmor’s daughter,
‘Tis she that cut down the forest.
Lugh’s foster-mother, men declare,
The place of this assembly (is) round Tailtiu.

(The Edginburgh Dinnshenchas" by Whitley Stokes, Folklore IV)

Given that Lughnasadh is a time of joyous celebration, it is surprising to learn that Lughnasadh was originally funerary games for Tailtiu, the foster mother of the Irish God Lugh, who died clearing forested land for cultivation. This information is given in a Medieval poetic anthology known as the  Metrical Dindshenchas:

"Long was the sorrow, long the weariness of Tailtiu, in sickness after heavy toil; the men of the island of Erin to whom she was in bondage came to receive her last behest. She told them in her sickness (feeble she was but not speechless) that they should hold funeral games to lament her - zealous the deed."

The story is also recorded within one of the other most important sources for Irish lore, the Lebor Gabala Erren:

"So Tailltiu died in Tailltiu [modern Telltown], and her name clave thereto and her grave is from the Seat of Tailltiu north-eastward. Her games were performed every year and her song of lamentation, by Lug [Lugh]."

According to legend, she was the daughter of a king of Spain (Magmor) and the wife of King Eochaid mac Eirc, son of Erc, son of Rinnal (Eochu Garb son of Dui Dall, depending on version of the myth).  It is also said that Tailtiu brought enormous wealth to their marriage in her dowry. As one of a number of Gods associated with the seasonal festival of Lughnasadh in various places, it is at Telltown (which is named after her - that being "Taillten") that the inauguration of Lughnasadh as the funerary games of Tailtiu was held.

As for her name, it is believed that the name "Tailtiu" is cognate with that of the Roman Goddess Tellus, who is herself the land, and it is suggested that it originates from the word 'Talantiu', meaning 'The Great One of the Earth'.  

Sources and further Reading:

Many Gods

Ancient Worlds