Irish harvest goddess and eponym of Telltown in Ireland. Daughter of Mag Mor king of Spain and wife of Eochaid the Rough son of Duach, she was a Fomorian queen who lived with the Tuatha de Dannan. This was not unusual, as the Fomorians and de Dannans seemed to intermarry with regularity, despite their later enmity.
She is the foster-mother of Lugh Lamhfada. Lugh's mother Eithne daughter of Balar was a Formorian, while his father Cian was Tuatha de Dannan. So on Emhain Abhlach, Lugh was raised by Tailtiu and Manannan mac Lir.
After the Second Battle of Magh Turedh, Lugh established funeral games in honor of Tailtiu, who died in the war between the de Dannans and the Fomorians. These funeral games were called "Lughnasadh"--the festival of Lugh:
Her games were performed every year and her song of lamentation, by Lugh. With gessa and feats of arms were they performed, a fortnight before Lugnasad and a fortnight after: under dicitur Lughnasadh, that is, the celebration (?) or the festival of Lugh.
This festival was held fifteen days before through fifteen days after August 1, which was the height of the festival. This was one of the chief holy days of the Irish (and presumably Celtic) year, the first harvest celebration of the year, and the last major holiday before Samhain.
Source: Lebor Gabal Erenn. The Book of Invasions Translated from the Book of Leinster.
Edited by R.A.S. Macalister for The Irish Texts Society.