Aos Sí
Aos sí (pronounced [iːsˠ ˈʃiː]; older form: aes sídhe [eːsˠ ˈʃiːə]) is the Irish name for a supernatural race in Celtic mythology – daoine sìth in Scottish Gaelic – comparable to fairies or elves. They are said to descend from the Tuatha Dé Danann, meaning the "People of Danu", depending on the Abrahamic or pagan tradition.The aos sí are said to live underground in fairy forts, across the Western sea, or in an invisible world that co-exists with the world of humans. This world is described in the Lebor Gabála Érenn as a parallel universe in which the aos sí walk among the living. 
In modern Irish the people of the mounds are also called daoine sí; in Scottish Gaelic they are called daoine sìth (in both cases, it means "people of the fairy mound"). They are variously said to be the ancestors, the spirits of nature, or goddesses and gods.

Etymology
In the Irish language, aos sí means "people of the fairy mounds", as "sídhe" means the Otherworldly mounds or hills. In modern Irish, the word is sí; in Scottish Gaelic, sìth; in Old Irish, síde, and the singular is síd.By the time of the Celtic Revival, when the “Fairy Faith” became a topic for English and English-language authors, sidhe in its various forms, with various meanings, became a loanword into English and took on a variety of, often inaccurate, meanings.

The sídhe
The Sídhe are the hills or tumuli that dot the Irish landscape. In modern Irish the word is sí; in Scottish Gaelic, sìth; in Old Irish síde and the singular is síd. In a number of later, English-language texts, the word sídhe is incorrectly used both for the mounds and the people of the mounds. For example W. B. Yeats, writing in 1908, referred to the aos sí simply as "the sídhe". However sidh in older texts refers specifically to "the palaces, courts, halls or residences" of the otherworldly beings that supposedly inhabit them. The fact that many of these sídhe have been found to be ancient burial mounds has contributed to the theory that the aos sí were the pre-Celtic occupants of Ireland.
David Fitzgerald conjectured that the word sídh was synonymous with "immortal," and is compared with words such as sídsat "they wait/remain," síthbeo "lasting," sídhbuan "perepetual," and sídhbe "long life." In most of the tales concerning the sí a great age or long life is implied.

In Irish folklore
In many Gaelic tales, the aos sí are later, literary versions of the Tuatha Dé Danann ("People of the Goddess Danu")—the deities and deified ancestors of Irish mythology. Some sources describe them as the survivors of the Tuatha Dé Danann who retreated into the Otherworld when fleeing the mortal Sons of Míl Espáine who, like many other early invaders of Ireland, came from Iberia. As part of the terms of their surrender to the Milesians, the Tuatha Dé Danann agreed to retreat and dwell underground. (In later interpretations, each tribe of the Tuatha Dé Danann was given its own mound.) Geoffrey Keating, an Irish historian of the early 17th century, equates Iberia with the Land of the Dead, providing a possible connection to the aos sí.
In folk belief and practice, the aos sí are often appeased with offerings, and care is taken to avoid angering or insulting them. Often they are not named directly, but rather spoken of as "The Good Neighbours", "The Fair Folk", or simply "The Folk". The most common names for them, aos sí, aes sídhe, daoine sídhe (singular duine sídhe) and daoine sìth mean, literally, "people of the mounds" (referring to the sídhe). The aos sí are generally described as stunningly beautiful, though they can also be terrible and hideous.
Aos sí are seen as fierce guardians of their abodes —whether a fairy hill, a fairy ring, a special tree (often a hawthorn) or a particular loch or wood. It is believed that infringing on these spaces will cause the aos sí to retaliate in an effort to remove the people or objects that invaded their homes. Many of these tales contribute to the changeling myth in west European folklore, with the aos sí kidnapping trespassers or replacing their children with changelings as a punishment for transgressing. The aos sí are often connected to certain times of year and hours; as the Gaelic Otherworld is believed to come closer to the mortal world at the times of dusk and dawn, the aos sí correspondingly become easier to encounter. Some festivals such as Samhain, Bealtaine and Midsummer are also associated with the aos sí.

Alternate names in Irish folklore
The Aos sí are known by many names in Ireland, among them:
Aingil Anúabhair: "Proud angels"
Daoine Uaisle': "The noble folk"
Daoine maithe: "Good people"
Deamhna Aerig: "Air demons"
Dream Anúabhair: "Excessively proud [people]"
Sídhfir: "fairy men"
Sídheógaídhe: "Little fairies"
Slúagh Cille:  "Host of the churchyard"
Slúagh na Marbh: "Host of the dead"
Slúagh Sídhe: "fairy host"
Slúagh-Sídhe-Thúatha-Dé-Danann: "fairy host of the Túatha Dé Danann"
na Uaisle: "The noble" or "The gentry"

Daoine maithe
Daoine maithe is Irish for "the good people", which is a popular term used to refer to the fairies in Irish folklore. Due to the oral nature of Irish folklore the exact origins of the fairies is not well defined. There are stories enough to support two possible origins. The fairies could either be fallen angels or the descendants of the Tuatha Dé Danann; in the latter case this is equivalent with Aos Sí. In the former case, it is said that the fairies are angels who have fallen from heaven, but whose sins were not great enough to warrant hell.They are generally human-like, though there are exceptions such as the púca and the mermaid. The defining features of the Irish fairies are their supernatural abilities and their temperament. If treated with respect and kindness, Irish fairies can be quite benevolent; however, if they are mistreated they will react cruelly.

Types
The banshee or bean sídhe (from Old Irish: ban síde), which means "woman of the sídhe", has come to indicate any supernatural woman of Ireland who announces a coming death by wailing and keening. Her counterpart in Scottish mythology is the bean sìth (sometimes spelled bean-sìdh). Other varieties of aos sí and daoine sìth include the Scottish bean nighe: the washerwoman who is seen washing the bloody clothing or armour of the person who is doomed to die; the leanan sídhe: the "fairy lover"; the cat sìth: a fairy cat; and the Cù Sìth: fairy dog.
The sluagh sídhe — "the fairy host" — is sometimes depicted in Irish and Scottish lore as a crowd of airborne spirits, perhaps the cursed, evil or restless dead. The siabhra (anglicised as "sheevra"), may be a type of these lesser spirits, prone to evil and mischief. However, an Ulster folk song also uses "sheevra" simply to mean "spirit" or "fairy".

List
Creideamh Sí
Creideamh Sí is Irish for the "Fairy Faith", a collection of beliefs and practices observed by those who wish to keep good relationships with the aos sí and avoid angering them. The custom of offering milk and traditional foods—such as baked goods, apples or berries—to the aos sí has survived through the Christian era into the present day in parts of Ireland, Scotland and the diaspora. Those who maintain some degree of belief in the aos sí also are careful to leave their sacred places alone and protect them from damage through road or housing construction.

See also
Enchanted Moura
Edmund Lenihan
Ailill (Old Irish for "elf")
Fairy riding
Fir Bolg
Jinn (Arab mythical being)
Otherworld
Strontian

References
Primary sources
Lebor Gabála Érenn (The Book of Invasions) in Lebor Laignech (The Book of Leinster)
Annála na gCeithre Máistrí (The Annals of the Four Masters)
Leabhar Bhaile an Mhóta (The Book of Ballymote)
Lebor na hUidre (The Book of the Dun Cow)
Leabhar Buidhe Lecain (The Yellow Book of Lecan)
Leabhar (Mór) Leacain (The Great Book of Lecan)

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