The historical core of Brigid's life is frustratingly thin. She was born around 451, probably in Leinster, and was associated with Kildare ('Church of the Oak') from an early stage. The monastery she founded there became a major ecclesiastical center — Cogitosus, writing her earliest Life around 650, describes a grand church with elaborate decoration and a thriving double community of monks and nuns.

Brigid's foundation at Kildare was unusual in the early Irish church for the authority she exercised. Cogitosus describes her as governing both the women's and men's communities, and later tradition credited her with quasi-episcopal authority — a bishop was attached to her community, but it was Brigid, not the bishop, who was the true power. This has led some scholars to see Kildare as evidence for women's ecclesiastical authority in the early Celtic church, though others caution against reading too much from hagiographic sources.

What is clear is that Kildare, under Brigid's leadership and that of her successors, became one of the wealthiest and most influential churches in Ireland. The Book of Kildare, a lavishly illuminated gospel book now lost, was described by Giraldus Cambrensis in the twelfth century as a work of almost supernatural beauty.

Brigid died around 523. Her cult spread rapidly throughout Ireland and eventually across the Irish diaspora to Scotland, Wales, and the Continent. She was venerated at Bruges, where her relics were translated, and her feast on February 1 coincides with the ancient Irish festival of Imbolc — a coincidence that has fueled scholarly debate about the relationship between the saint and the goddess.

Brigid is the subject of extensive hagiographic tradition. According to Cogitosus and later Lives, she was born of a mixed marriage, vowed virginity, founded Kildare with astonishing speed, performed countless miracles (healings, multiplications of food, exorcisms), and established the perpetual flame. In Irish folklore, Brigid is often conflated with the pre-Christian goddess Brigid.