Aengus was a monk in Clonenagh, County Laois, renowned for his devotion to the saints. He composed two Martyrologies and was part of a reform movement in the late 8th and early 9th centuries that undertook more austere forms of prayer, penance and fasting, called the Culdee movement from Céilí Dé, “the companions and intimates of God” or "Spouses of God".
After his training, he was first a hermit at Dysert Beagh, and later at Dysert Enos, where he became well-known for his austerities of prayer, penance and fasting. So many people came to visit him that he sought refuge at the monastery of St Máelrúain of Tallaght.
Aengus collaborated with Máelrúain on The Martyrology of Tallaght, but later returned to Clonenagh, becoming an abbot and then a bishop. He also compiled his own calendar of Irish saints, known as a Féilire or Festilogium.
Aengus died in 824, and was regarded as one of the founders, along with Máelrúain, of the Céilí Dé (servant of God) movement. His feast day is celebrated on March 11.
More on the Culdees
"The holy men and women of the Culdean movement emphasised a direct gnosis of God, not qualified by an intermediary theological hierarchy. Rather like the Essenes, who lived in Jesus' times, their contemplative life combined with a ‘right way’ of being in the world. One of their chief occupations seemed to have been healing, with the use of herbs they cultivated. Toward the end of their lives some members curbed their secular activities and lived in makeshift
monasteries that were either caves, or beehive cells constructed from large stones." culdeebook.PDF (earthwise.me)
References
Óengus the Culdee, Hermit | Citydesert (wordpress.com)
Óengus of Tallaght - Wikipedia
CATHOLIC ENCYCLOPEDIA: St. Aengus (The Culdee) (newadvent.org)