Tlachtga
Book of Ballymote: RIA MS 23 P 12, col. 406

Why was Tlachtga so called? Not difficult. Tlachtga was the daughter of Mog Roith, son of Fergus. Three sons of the magician Simon raped her. She went with her father to learn the magic arts practised anywhere in the world. And it was she who made for Trian the Rolling Wheel, the stone in Forcathu and the Pillar in Cnamchaill. She came from the East and brought with her these things until she reached the hills of Tlachtga. It was there she bore three sons, Doirb, Cumma and Muach, who gave their names to three regions. As long as their names are remembered in Ireland the land will not be visited by vengeful strangers. It is said of Tlachtga:

Tlachtga Hills, splendid and high, 
Foreboding doom to a great, unswerving king
Before the step which Tlachtga... took, 
The daughter of King Roth's clever votary. 
Mog Roith, the son of Fergus Fal, 
The kingly and noble son of Ross. 
Cacht, the daughter of the quarrelsome Catmend
Was his colourful arid noble mother. 
Roth, son of Rigoll was his fosterer. 
This is why the name 'Mog Roith' was given him. 
Two sons of Mog: Buan and Fer-Corb, 
Were successful over armies in deeds of liberation. 
She [Cacht] was the [foster] mother of the handsome sons 
Of Der-Droighen, dark, strong and active, 
And the real mother of Cairpre [Lifechair]. 
It is certain that he deceived the Hui-Bairdne. 
The daughter of Mog hosted with thousands, 
Tlachtga, the chosen - not that she was without feelings 
To accompany her great and noble father, 
To noble Simon of sevenfold splendour. 
Three sons had Simon pleasing to look upon: 
Sorrowful her struggle with their devilry.
... [text missing]... powerful. 
Theirs was a powerful family, vehement and resilient. 
The sons grew passionate Towards Tlachtga at the same time, 
They flowed into her body - it is no lie [making] descendants of beauty and lineage.
For Trian it was no honour 
Tlachtga Created the red and swiftly mobile wheel, 
Together with the great and noble Mog, 
And with Simon of sevenfold splendour. 
She brought with her wise sayings; 
She left the moving wheel, 
The finished stone of Forcarthu she left, 
And the pillar in Cnamchaill. 
Whoever sees it will become blind, 
Whoever hears it will become deaf, 
And anyone who tries to take a piece of the 
Rough spoked wheel will die... 
After the woman came from the East, 
She gave birth to three sons after hard labour. 
She died, the light and lively one. 
This urgent, unconcealable news was to be heard. 
The names of the sons were of great import... 
Muach and Cuma and Doirb the noble. 
The crowd... [text missing)... 
because it is appropriate that they shall hear it: 
That as long as over the stately Banba [Ireland] 
The names of the three sons are remembered 
As the truthful story tells... 
No catastrophe will befall its inhabitants. 
The hill where this woman from the East is buried, 
To surpass all other women, This is the name it was given: 
The Hill of Tlachtga.

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SOURCES:
Gwynn, Edward. "Tlachtga." Metrical Dindshenachas IV. Dublin: Dublin Institute for Advanced Studies. 1906, 1991. p. 189-191.

Matthews, John, and Caitlin. The Encyclopedia of Celtic Wisdom. NY: Element, 1994.

Müller-Lisowski, Käte. "Texte zur Mog Ruith Sage". Zeitschrift für Celtische Philologie. volume 14, Halle/Saale, Max Niemeyer (1923) page 157-163.