Gwenllian's mother, Eleanor de Montfort died during childbirth, or shortly after, on 19 June 1282.
The etymology of the name is unclear, with the personal name "Helen" or "Eleanor" being one possibility, perhaps after Eleanor de Montfort (d. 1282), princess of Wales and wife of Llywelyn ap Gruffudd.
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In 1275, after the death of his mother at Montargis Abbey, Amaury, by then a Papal Chaplain, accompanied his younger sister Eleanor de Montfort on a winter sea voyage to Wales and her new husband, Llywelyn ap Gruffydd (the grandson of Llywelyn Fawr).
A cleric, he worked in the papacy before accompanying his sister Eleanor de Montfort to Wales for her marriage to Llywelyn ap Gruffydd, Prince of Wales.