Why do we offer to the Gods and daoine sidhe?
Not hard to say. When the sons of Mil defeated the Tuatha De Danann the land was divided between the two, with the Milesians taking what was above the earth and the Tuatha De taking what was beneath it. But soon the Gaels found that their cows would not give milk and their crops would not grow, because the Tuatha De destroyed all the produce. The sons of Mil reached an agreement with an Dagda that they would offer a portion of their harvest in exchange for the friendship of the Tuatha De. After that time offerings of milk and grain were made to the Gods and later to the daoine sidhe as well, to ensure a good harvest and abundant milk. Today we offer to receive blessing and luck in our lives.
Who divided the Sidhe?
Not hard to say. When the Tuatha De went into the sidhe, the hollow hills, it was necessary to decide who should go to live where. An Dagda was the High King over the Tuatha De then and it fell to him to divide up the sidhe; with Manannan advising him, he chose a hill for each of the Gods. In this way each of the Tuatha De found a home within the sidhe.
How did Angus Og come to posses Brugh na Boinne?
Not hard to say. When an Dagda divided up the sidhe he took several places for himself including Síd Leithet Lachtmaige, Oí Asíd, Cnocc Báine, and Brú Ruair; and Brugh na Boinne was also his. His son Angus Og came to his father and asked what place should belong to him, but an Dagda said that all the sidhe had been claimed and none were left for Angus. So Angus asked if he might have a night and day in Brugh na Boinne and, seeing no harm in it, an Dagda agreed.
After the allotted time had passed an Dagda came to Brugh na Boinne and asked Angus to leave as they had agreed, but Angus replied, "Is not all time divided into a day and a night? So that is how long I have been granted this place."
Then an Dagda left and the Brugh belonged always to Angus Og.
The Book of Leinster version http://tairis-cr.blogspot.co.uk/2011/03/de-gabail-in-t-sida-in-so-sis.html
The Book of Fermoy version http://www.maryjones.us/ctexts/fosterage.html
Fairy Faith in Celtic Countries