In Irish mythology, Manannan mac Lir was a sea and weather god. He was one of the Tuatha Dé Danann, though he was considered older than them. His father was Lir and his wife was Fand. He ruled over the Blessed Isles as well as Mag Mell, the underworld.
Manannan had many magical items. He gave Cormac mac Airt his magic goblet of truth; he had a ship that did not need sails named "Wave Sweeper"; he owned a cloak that granted him invisibility; a flaming helmet; and a sword named "Answerer" that could never miss its target. He also owned a horse called "Enbarr of the Flowing Mane" which could travel over water as easily as land.
Manannan left his wife, Fand, and she fell in love with Cuchulainn. Since a mortal and fairy (Fand was Queen of the fairies) could not stay together without destroying the fairy, Manannan erased their memories of each other.
In the Isle of Man, Manannan mac Lir was known as Mannan. On Midsummer Eve, people offered green grass to Mannan-beg-mac-y-Leir and prayed for blessings in seafaring and fishing. He was believed to be a magician who could make an illusory fleet from pea shells in order to discourage would-be invaders.
Manannan may actually have been a real person - an Irish prince named Orbsenius who was hailed for his abilities at navigating the Irish sea as a merchant.
While Manannan was not personally in control of the Three-headed Dragon, he enjoyed watching its path of destruction across the lands on its way to the Kingdom of Daventry from afar, because of his hate of humanity.
Manannan was attempting to shock the giant, and the Giant was warning him not to do it again, or their would be consequences.
Manannan was a celtic god son of Lir (usually known as Manannan MacLir) and god of sea and weather.
And Manannan said he would give him no help, for he had told him at the time he brought Deidre away that she would be the cause of the breaking up of his kingdom, and he took her away in spite of him.
Manannan had good hounds one time, but they went hunting after a pig that was destroying the whole country, and making a desert of it.
But Manannan sent Fer Ferdiad, of the Tuatha de Danaan, that was a pupil of his own and a Druid, in the shape of a woman of his own household, and he went where Tuag was, and sang a sleep-spell over her, and brought her away to Inver Glas.