The Culdees (Irish: Céilí Dé, lit. "Companions of God") were members of ascetic Christian monastic and eremitical communities of Ireland, Scotland, and England in the Middle Ages. The etymology of the term, the persons designated by it, their origin, their doctrines, the rule or rules under which they lived and the limits of their authority and privileges have all been matters of controversy. All admit, however, that (in the beginning at all events) the Culdees were separated from the mass of the faithful, that their lives were devoted to religion and that they lived in community. Appearing first in Ireland and subsequently in Scotland, attached to cathedral or collegiate churches, they lived in monastic fashion though not taking monastic vows. From the 12th century Scottish and Irish Christianity was regulated on the Roman pattern and in the process the Culdees also lost any distinctiveness they may formerly have had and were brought under canonical rule.
According to Philip Schaff, the term Culdee or Ceile De, or Kaledei, first appears in the eighth century. While "giving rise to much controversy and untenable theories", it probably means servants or worshippers of God. It was applied to anchorites, who, in entire seclusion from society, sought the perfection of sanctity. They afterwards associated themselves into communities of hermits, and were finally brought under canonical rule along with the secular clergy, until at length the name of Culdee became almost synonymous with that of secular canon.
In the course of the 9th century, nine places in Ireland are mentioned (including Armagh, Clonmacnoise, Clones, Devenish and Sligo) where communities of Culdees were established.
Óengus the Culdee lived in the last quarter of the 8th century and is best known as the author of the Félire Óengusso "the Martyrology of Óengus". Maelruan, under whom Oengus lived, drew up a rule for the Culdees of Tallaght that prescribed their prayers, fasts, devotions, confession and penances, but we have no evidence that this rule was widely accepted even in the other Culdean establishments. Fedelmid mac Crimthainn king of Munster (820-846) was said to have been a prominent Culdee.