Céli Dé or culdees, as they are often called, appear about midway in the timeline of early medieval Irish monasticism. They emerged in the second half of the eighth century, some three hundred years after St Patrick began promoting the religious life to the Irish in the fifth century and a little more than three hundred and fifty years before the introduction of foreign religious orders in Ireland in the twelfth century. Céli Dé were religious, as the meaning of their name, ‘clients of God’, suggests, and in the view of most modern scholars who have written on the topic they were also strictly ascetic.

While scholars have identified a number of early medieval Irish ecclesiastics as céli Dé, such as Dublitter, abbot-bishop of Finglas (ob. 796), Elair, anchorite of Loch Cré (ob. 807), and Mael Díthruib, anchorite and ecclesiastical scholar of Terryglass (ob. 840), the most frequently mentioned is Mael Ruain, abbot-bishop of Tallaght (ob. 792). His community is believed to have produced several Old Irish texts, such as the Old Irish Penitential and Félire Óengusso (‘Martyrology of Óengus’), both likely written in the first half of the ninth century. The most informative witness of the céli Dé is a consuetudinal tract purporting to contain the teachings of Mael Ruain, apparently as recollected by his disciple Mael Díthruib. It was written in the first half of the ninth century by an anonymous monk who was personally acquainted with Mael Díthruib and may have belonged to the community of Tallaght.