Mabon: Mabon son of Modron ("young man" son of "mother goddess") was a hunter-god. He was stolen from his mother at three days, and lived in Annwn, whence he was rescued by Culhwch (and Arthur) as an adult. He was ever-young as a result of this sojourn. He assisted Culhwch in the quest for Olwen. Culhwch's stepmother wished him to marry her own daughter. When he demurred, she cursed him to marry no one but Olwen, daughter of Ysbaddaden. He has the power to make a land flourish or waste away. He enlisted the aid of Arthur to search for Olwen. When they found her, she said her father would not permit her to marry for that would cause his own death. Nonetheless, Culhwych overcame many trials and eventually married her.

Mag Mell (Tir na n-Og): In Celtic mythology, Mag Mell ("plain of joy") is a paradise were the deceased live. In contrast with other realms of the dead, Mag Mell is described as an idyllic island, or as a wonderful place at the bottom of the ocean. This realm of the dead is ruled by king Tethra of the Fomorians, a race of giants. But also the sea god Manannan mac Lir of the Tuatha Dé Danann is believed to rule there. Another name for this place is Tir na n-Og ("land of youth").

Mag Tuireadh: In Irish/Celtic mythology, the place where the two battles took place between the Fomorians and the Tuatha Dé Danann. In the first battle the Fomorians were driven away, but in the second battle, a true slaughter, the Fomorians were totally defeated and they left Ireland for good.

Manannan mac Lir: The Irish god of the sea and fertility, who forecasts the weather. He is older than the Tuatha Dé Danann, yet was considered to be one of them. He is the son of Lir and his name means "Manannan Son of the Sea". His wife is Fand and he is the foster-father of many gods, including Lugh. He is the guardian of the Blessed Isles, and the ruler of Mag Mell. Manannan has a ship that follows his command without sails; his cloak makes him invisible; his helmet is made of flames and his sword cannot be turned from its mark. He is described as riding over the sea in a chariot. His Welsh equivalent is Manawydan ap Llyr. He is also called Barinthus.

Manawydan: Manawydan ap Llyr, son of Llyr and Penarddun and brother of Branwen and half brother of Nisien and Efnisien. Manawydan was a scholar, a magician, and a peaceful man. But when Bran learned of the slavery imposed upon his sister Branwen by her Irish husband Matholwch, he joined the expedition to rescue her. Matholwch was terrified at the sight of a forest approaching Ireland across the sea: Bran's navy, and Bran himself wading through the water. He sued for peace, they built a house big enough for Bran, and Matholwch agreed to settle the kingdom on Gwern, his son by Branwen. Some Irish lords objected, and hid themselves in flour bags to attack the Welsh. But Efnisien, scenting Irish treachery, cast them into the fire, and then cast Gwern himself in (avoiding the geas against shedding kinsmen's blood thereby). A war broke out, and the Irish replenished themselves through the cauldron. Efnisien, repenting, sacrificed himself by feigning death and being thrown into the cauldron, which he then broke, dying in the process. Only seven Welshmen survived, and Bran was fatally wounded. His head, which remained alive and talking, was returned to Wales and buried, and soon afterwards Branwen sailed to Aber Alaw and died. Manawydan inherited after Bran's death, but surrendered the kingdom to his cousin to avoid warfare. He married Rhiannon, widow of Pwyll of Dyfed and mother of Pryderi, and went to live there. One day, all of Dyfed turned into a wasteland, and only Rhiannon, Manawydan, Pryderi, and his wife Cigfa, were spared. Manawydan and Pryderi out hunting followed an enormous white boar into a caer, where Pryderi saw a golden bowl; when he touched it, he was enspelled. Rhiannon went after him and fell under the same spell; the caer then vanished, taking them with it. Manawydan, turning to farming, lost his crops to an army of mice which were clearly faery mice. He managed to catch one, and despite being begged by important passersby (priests in the Christianized version) threatened to hang it. Finally, Llwyd appeared and confessed to being behind all the trouble, in vengeance for Gwawl, Rhiannon's original suitor.

Mannan: The Manx (Isle of Man) counterpart of the Irish sea-god Manannan mac Lir. On Midsummer Eve the people used to carry green meadow grass to the top of Barule in payment of rent to Mannan-beg-mac-y-Leir. People also used to pray to him for a blessing on their boats and a good catch. It was generally believed that Mannan was a great magician who could create an illusory fleet out of pea-shells and stick, to discourage an invasion of the island.

Maponos: The Celtic god of youth. Math Mathonwy: The Welsh god of sorcery, brother of the goddess Don

Matres (Matronae): Celtic mother goddess of Gaul.

Midir: The Irish/Celtic ruler of Mag Mor, the underworld. He is a son of the goddess Danu. When he was hit by a twig from a hazel tree and lost one of his eyes, the healer god Dian Cecht placed the healed eye back in its socket.

Modron: A Welsh goddess, daughter of Avalloc, derived from the Celtic goddess Matrona. She is regarded as a prototype of Morgan (from Arthurian Legend).

Mog Ruith: The one-eyed Celtic/Irish god of the sun who rides through the sky in a shining bronze chariot, or who flies through the sky like a bird. The word ruith is possibly derived from the Irish roth, meaning "wheel" (representing the sun).

Morrigan (Morrigu): . Click For The Full Discription

Murigen: A minor Irish lake goddess, probably another form of the Morrigan.