In Irish, Scots and Manx mythology, Manannán mac Lir is the god of the sea. He is often seen as a psychopomp, and considered to have strong connections to the Otherworld islands of the dead, the weather, and the mists between the worlds. He is usually counted as one of the Tuatha Dé Danann, although most scholars consider him to be of an older race of deities.
On the Isle of Man, Manannán mac Lir is known as Mannan-beg-mac-y-Leir (beg=small) The name Manannán derives from an earlier name for the Isle of Man, and his patronymic mac Lir may have been metaphorical and meant "son of the sea (Ler)": consequently, he is probably unrelated to the character Lir of the well known story of the Children of Lir.
His Welsh equivalent is Manawydan ap Llyr. In the “Yellow Book of Lecan”, there are said to be "four Manannans". The name given for the "first Manannan" is "Manandan mac Alloit, a Druid of the Tuath De Danann, and in the time of the Tuath De Danann was he. Oirbsen, so indeed, was his proper name.... Oirbsen over the land, so that from him (is named) Loch Oirbsen. This was the first Manannan.", (Skene, William F. The Four Ancient Books of Wales "Chapter VI. Manau Gododin and the Picts" online
- English: Manannan mac Lir
- Manx: Mannan beg mac y lir (“little Manannan son of the sea”)
- Welsh: Manawydan fab Llyr
- There are numerous spelling variations including most commonly: Mannanan, Mananan, Lyr, and Llyr
Manannán features in many Celtic myths, although he only plays a prominent role in some of them.
In the tale "His Three Calls to Cormac", Gregory, Lady Augusta. "Part I Book IV: His Three Calls to Cormac" in Gods and Fighting Men (Colyn Smyth, Buckinghamshire, 1903) online Manannán tempts the Irish King Cormac mac Airt with treasure in exchange for his family. Cormac is led into the otherworld and taught a harsh lesson by Manannán, but in the end his wife and children are restored to him, and Manannán rewards him with a magic cup which breaks if three lies are spoken over it, and is made whole again if three truths are spoken.
The tale "Manannan at Play" features the god as a clown and beggar who turns out to be a harper.  Manannán, here in his trickster guise, plays a number of pranks, some resulting in serious trouble, but by the end of the tale he once again sets everything to right.
In the[Ulster Cycle tale, Serglige Con Culainn ("The Sick-bed Of Cuchulain") Manannán left his wife, Fand, and she fell in love with Cúchulainn. But when Fand saw that Cúchulainns jealous wife, Emer was worthy of him, she decided to return to Manannán, who then shook his magical cloak of mists between Fand and Cúchulainn, that they may never meet again. 
Manannán also prophesied to Bran, in the “Voyage of Bran”, that a great warrior would be descended from him. The 8th century saga Compert Mongáin recounts the deeds of a legendary son, Mongán mac Fiachnai, fathered by Manannán on the wife of Fiachnae mac Báetáin.
Manannán has strong ties to the Isle of Man, where he is referenced in a traditional ballad  as having been the nation's first ruler. On Midsummer, the Manx people offer bundles of reeds, meadow grasses and yellow flowers to Manannán in a ritual "paying of the rent", accompanied with prayers for his aid and protection in seafaring and fishing. He is also believed to have been a magician who could make an illusory fleet from sedge or pea shells in order to discourage would-be invaders.  
According to the Book of Fermoy, a Manuscript of the 14th to the 15th century, "he was a pagan, a lawgiver among the Tuatha Dé Danann, and a necromancer possessed of power to envelope himself and others in a mist, so that they could not be seen by their enemies." It was by this method that he was said to protect the Isle of Man from discovery.
Manannán was associated with a "cauldron of regeneration". This is seen in the tale of Cormac mac Airt, among other tales. Here, he appeared at Cormac's ramparts in the guise of a warrior who told him he came from a land where old age, sickness, death, decay, and falsehood were unknown (the Otherworld was also known as the "Land of Youth" or the "Land of the Living").
As guardian of the Blessed Isles as well as Mag Mell he also has strong associations with Emhain Abhlach, the Isle of Apple Trees, where the magical silver apple branch is found. To the Celts, the Blessed Isles that lie beyond the sea are the gateways to the Otherworlds, where the soul journeys to after death. Manannán is the guardian of these gateways between the worlds. He is the Ferryman, who comes to transport the souls of the dead through the veils.
Manannán 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 of mists that granted him invisibility, a flaming helmet, and a sword named Fragarach ("Answerer" or "Retaliator") 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. In some sources he is described as driving his chariot over the sea as if over land, and through fields of purple flowers.
According to Táin Bó Cúailnge (the Cattle Raid of Cooley), his wife is the beautiful goddess, Fand ("Pearl of Beauty" or "A Tear" - later remembered as a "Fairy Queen", though earlier mentions point to her also being a sea deity). Other sources say his wife was the goddess Áine, though she is at other times said to be his daughter. As the Celtic concept of "marriage" was not necessarily monogamous or permanent, both goddesses could have been his "wives", and he is connected romantically with others as well. Manannán had a daughter, whose name was Niamh of the Golden Hair. It is also probable that another daughter was Cliodna, but sources treat this differently. Either way, she is a young woman from Manannán's lands, whose surname is "of the Fair Hair". Mongan is a late addition to the mac Lir family tree. The historical Mongan was a son of Fiachnae mac Báetáin, born towards the end of the 6th century. According to legend Fiachnae, who was at war in Scotland, came home with a victory because of a bargain made with Manannán (either by him, or by his wife) to let Manannán have a child by his wife. This child, Mongan, was supposedly taken to the Otherworld when he was very young, to be raised there by Manannán. The Compert Mongáin tells the tale.
Despite not being the biological father of many children, Manannán is often seen in the traditional role of foster father, raising a number of foster children including Lugh of the great hand and the children of Deirdre.
- Today there is a museum in the City of Peel on the Isle of Ma] named the House of Manannan as well as an annual celebration of the arts The Mananan Festival.
- The traditional of offering bundles of reeds on the Isle of Man is still practiced as an opening ceremony of Tynwald. 
- Folklore of the Isle of Man, A.W.Moore, 1891 online
- Gods and Fighting Men, Lady Gregory; 1904 online
- The Manx Notebook online
- The Temple of Manannan: A Neopagan site