Kildare, where Brigid gathered a group of women, and later men, into a community of nuns and monks, had formerly been a pagan shrine where a sacred fire was kept perpetually burning. Instead of stamping out the fire, Brigid and her nuns kept it going, but interpreted it in a Christian way. This was in keeping with the general process in Ireland whereby Druidism was supplanted by Christianity with minimal opposition. Many Druids felt that their own beliefs were a partial and tentative insight into the nature of God, and many recognized in Christianity what they'd been looking for.
Perhaps my favorite bit of lore about the "Mary of the Gaels" is the prayer attributed to St. Brigid: