the grianÁn of aileach
Wikipedia states that the name Dagda is believed to come from Proto-Celtic: Dagodeiwos, "the good god" or "the great god", but may ultimately be derived from the Proto-Indo-European Dhagho-deiwos "shining divinity", the first element being cognate with the English word "day", and possibly a byword for a deification of a notion such as "splendour". This etymology would tie in well with the Dagda's mythic association with the sun and the earth, with kingship and excellence in general. Dhago-deiwos would have been inherited into Proto-Celtic as Dago-deiwos, thereby punning with the Proto-Celtic word dago-s "good".
Be that as it may, the Dagda was known as Ruad Rofhessa ("mighty one/lord of great knowledge"). He owns a magic staff, club, or mace (the lorg mór or lorg anfaid) which kills with one end and brings to life with the other, a cauldron (the coire ansic) which never runs empty, and a magic harp (uaithne) which can control men's emotions and change the seasons. The Dagda is said to dwell at Ireland's most famous megalithic site, Brú na Bóinne (i.e. Newgrange in the Boyne Valley UNESCO World Heritage Site), whose passage is illuminated by the light of the rising sun at dawn on Winter Solstice.
According to ancient oral history that was first written down in the 6th century CE, the Dagda built the Grianán of Aileach to protect the grave of his son Aedh. The first King of the Tuatha Dé Danann, Nuada Aigetlám (Nuada of the Silver Hand) is also said to be buried here, as are many O’Neill Kings from the middle ages.
The original Neolithic cairn was incorporated into a surrounding double ring-fort in the Bronze or Iron Age. In Celtic times it was known as the hibernation palace of the Sun Goddess, Graine, and the capital of the Kindgom of Aileach. In 674, Fínsnechta Fledach, King of Ireland, destroyed the existing hillfort. In 937 it was attacked again by Viking raiders. In 1006, the High King Brian Boru probably came to visit. In 1101, the King of Munster Muirchertach Ua Briain completely vandalised and destroyed the site, ordering his army to remove and scatter all the stones of the orginal cairn. By the 12th century, the Kingdom was repeatedly attacked and lost a lot of territory to the Anglo-Norman invaders. The current structure —which measures 24m diameter with 5m high walls— is thought to have been re-built in the 6th or 7th century by the Northern O'Neill kings, who occuplied the site from c. 789 to about 1050 CE. The site was surveyed by the archaeologist George Petrie in the 1830s. Substantial restoration work was carried out by the archaeologist Walter Bernard in 1874-1878, with help from hundreds of volunteers from the surrounding community. Today's structure preserves the solar alignment of the original cairn.