Little is known of her early life, but we do know Saint Girmonia was the daughter of a pagan Irish chief and converted to Christianity some time around the age of twelve. As a part of her conversion, she also took a vow of perpetual virginity, much to the anger of her father. Her father demanded she marry and would not accept her refusal.
To escape her father, she fled to France and became a solitary in the forest of Thierache near Picardy. Here she devoted her life to God, until agents sent by her father found her. When she again refused to return to Ireland and marry, she was beheaded sometime around 572. The local community built a chapel over her grave and the town of LaChapelle grew around the church. It was known to be a place of frequent miracles. On September 7, 1231, her relics and relics of Saint Proba (Preuve) were enshrined at Les Quielles.