In November 1200, Margaret married Walter de Lacy, Lord of Trim Castle in County Meath, Ireland, and Ludlow Castle in Shropshire. He also owned many estates and manors in Herefordshire including Ewyas Lacy. He was later appointed Sheriff of Hereford. It was an advantageous marriage as Walter and her father both held castles and lordships in the Welsh Marches as well as Ireland, and thus the two men looked after each other's interests in both places. Together Walter and Margaret had at least six children who included:
In 1208, Margaret's parents lost favour with their patron, King John of England, who seized all of the de Braose castles in the Welsh Marches. In order to escape from John's vindictive wrath, Margaret's mother, Maud and her eldest brother William fled to Ireland where they found refuge with Margaret and her family at Trim Castle. In 1210, however, King John sent an expedition to Ireland. Maud and William escaped from Trim but were apprehended on the Antrim coast while attempting to sail to Scotland. They were dispatched to England where they were both left to starve to death inside the dungeon of Corfe Castle, Dorset on the orders of King John. Walter de Lacy's estates were forfeited to the Crown as punishment for having harboured traitors inside his castle. By 1215, Walter and Margaret were back in the King's favour, and Walter's confiscated estates were restored to him. As a further token of John's favour, Walter was appointed Castellan and Sheriff of Hereford the following year, and Margaret obtained permission to found a religious house in memory of her mother. On 10 October 1216, eight days before his death, King John conceded three carucates of land in the royal forest of Aconbury, Herefordshire to Margaret for the construction of the Hospital of St. John. King John sent the instructions to her husband Walter by letters patent. Margaret's subsequent attempts to free her foundation from the control of the Hospitallers led her into a lengthy dispute which ultimately involved the Pope. Margaret died on an unknown date sometime after 1255. Her husband had died in 1241, leaving his vast holdings and lordships to their granddaughters by their son Gilbert, Margery de Lacy, and Maud de Lacy, Baroness Geneville.