Father Brien is a Christian hermit who lives in a cottage near the Sevenwaters forest. His dwelling also consists of a cave and can be reached by both cart or a more elaborate route through the forest, across a stream, and up a rock chimney.
He is described as "old" (in his fifties) with grey hair and of small stature. He is noted to have an aura of calmness and acceptance. It is mentioned that, despite his age, he climbs Ogma's Peak which demonstrates his stamina and strength.
He trades education and friendship for basic resources such as fish, herbs or bread. He forms a friendship based on the love of learning with Conor. He would teach many things such as the history of the people, reading and writing, and the language of British to Sorcha, Conor and Finbar which is vital further in the book.
Although his past is somewhat of a mystery, it is believed that Father Brien was once a warrior from Amorica but moved to Kells to be a scribe and monk. He is rather accepting and open-minded about other faiths and this is said to have caused him to leave the monastry of Kells and move to Sevenwaters. He prays blessings over the family and Lord Colum's fighting squad before they leave to campaign or fight.
His knowledge of herb lore is appreciated and accepted by the villagers. Sorcha has learned some of the fundamentals of her healing craft from Father Brien. They often work cooperatively to heal the sick in the village, Father Brien's strength to set bones and the like is complemented by Sorcha's herb lore.
Father Brien assists Finbar in smuggling Simon away from the tuath of Sevenwaters and attempting to heal him in his own residence. Because Simon does not respond positively to his healing treatments and prayers, Father Brien asks Sorcha to assist. It is this very task that begins Sorcha's journey. She is forced away from Simon but not before he creates a carving for her which his brother Red recognises later in the book.
Father Brien brings the tidings of Simon's disappearance to Sorcha. He also discloses some information to Sorcha regarding Lord Colum's past, how he met his wife Niamh and Conor's destiny to be a druid. Father Brien is not fond of Lord Colum's new wife, Lady Oonagh. He tries to discuss this matter with Lord Colum but he is not successful and is resigned to perform the marriage service.
When her brothers are turned into swans by the Lady Oonagh, Sorcha flees to Father Brien's cottage to find he has passed away. This is seen as a great loss to Sevenwaters. Sorcha is able to take some necessities from his cottage to help her on her way, but often reflects on her time with Father Brien. When her brothers find out about Father Brien's death, they suspect their step mother Lady Oonagh may have caused it through magic or poison. The cause of death is never disclosed throughout the series.
At the end of Daughter of the Forest, a pregnant Sorcha and her husband Red visit Father Brien's cottage and Sorcha reflects briefly on Father Brien and how, by healing Simon, she came to meet Red.