This tale was written in Old Gaelic (about the 9th century). Tuatha Dé Danann “The Peoples of the Goddess Danann” was a name used by the medieval Gaelic literati to refer to the old pagan gods, who were also called aos síde. This appellation played on the ambiguities of the word síd, which could mean both “peace” and “fairy mound.” This story, like many others, depicts rivalries between the gods as well as between the gods and the Gaels, who are referred to as the “sons of Míl” after a founder figure.
Dagán is an affectionate nickname for Dagda [“Good-god”] Mór the son of Eithlen, who is one of the main male gods; Mac Óg [“Young-Son”] is a by-name for the god Oenghus. The trick played by Oenghus on the Dagda exploits the fact that there is no indefinite article in the Goidelic languages: thus the “trick” phrase in Gaelic used by Oenghus could be translated as either “day and night” or “a day and a night” in English, and in fact both are used below in order to sound natural in translation.
The following is a revised translation by Michael Newton based on the edition in Vernam Hull, “De Gabáil in t-Sída (Concerning the Seizure of the Fairy Mound.” Zeitschrift fur Celtische Philologie 19 (1931): 53-58.
There was a wonderful king over the Tuatha Dé Danann in Ireland: his name was Dagán. He had great power, even though the land belonged to the descendants of Míl. The Tuatha Dé Danann used to destroy the crops and milk produced by the descendants of Míl, until they formed a pact with the Dagda. After that he protected their crops and milk.
At first, he had great power as king. He was the one who assigned the síd-mounds to the Tuatha Dé: Lug mac Ethnenn was assigned to Síd Rodrubán; Ogma was assigned to Síd Aircelltrai; the Dagda himself got Síd Leithet, Oí Síd, Cnoc Báine, and Brú Ruair, although it is said that he had Síd in Broga [Newgrange] from the beginning.
The Mac Óc [“Young Son”] was the foster-son of Midir of Brí Léith and of Nindid the seer. After everyone else was assigned a place, the Mac Óc went to the Dagda to get land.
“I have nothing for you,” said the Dagda. “All land has been assigned.”
“Give me just a day and a night in your own dwelling,” said the Mac Óc. And that was given to him.
After that, the Dagda said to him, “Off you go to your people. Your time is up.”
“It is a fact,” the Mac Óc said, “that all existence is made up of day and night and that is what you offered me.”
So Dagán left and the Mac Óc is still in the síd. A wonderful world is in it. There are three trees that bear fruit eternally, an immortal pig standing on its feet, a roasted pig, and a vessel containing a staggering drink, and they can never be depleted.