Fachtna mac Mongaig (Fachanan) was the founder of the monastery of Rosscarbery (Ros Ailithir), County Cork. He died around 600. His monastery became the principal monastery of west Cork, and later had a famous Scripture school known as the School of Ross. It flourished for three hundred years and survived in some form until the coming of the Normans to Ireland. It was the centre from which the Diocese of Ross developed.
Fachtna was born at Tulachteann, and was one of the pupils of Saint Ita. Before establishing the monastic school of Ross, he founded the monastery of Molana on an island in the Blackwater, near Youghal. He suffered for a time from blindness, from which he recovered at the intercession of Saint Ita's sister, who was about to give birth to Saint Mochoemog. Fachtna was revered as a "wise and upright man", with a great gift for preaching; Saint Cuimin of Connor said of him that he was "generous and steadfast, fond of preaching to the people and saying nothing that was base or displeasing to God".
His feast day was 14 August, a date now occupied by the memorial of Saint Maximilian Kolbe, and Saint Fachtna is now venerated, even in the Diocese of Ross, on 13 August.