Daoine maithe is Irish Gaelic for "the good people", which is a popular term used to refer to the fairies in Irish folklore. They are generally human like though there are exceptions such as the Puca or 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.
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. 
It is said that the fairies are angels who have fallen from heaven, but whose sins were not great enough to warrant hell. During the War of Heaven there were angels who refused to take either God's or Lucifer's side. For their lack of action they were banished from heaven. Some of them fell onto Ireland, some fell into the sea and some fell into hell.
Tuatha Dé Danann
The Tuatha Dé Danann were magical beings who occupied Ireland prior to the current inhabitants, the Milesians. A war was waged between these two groups for control of the island. In the end a truce was reached which gave the Milesians ownership of the land of Ireland and the Tuatha De Danan the ownership of the area below the land of Ireland.