She was born into a prominent and influential family that was heavily involved in the politics of the 13th century.
William de Valence was a French nobleman and Knight with close connections to the Crown, being a half-brother to Henry III of England and uncle to Edward I.
[1] Thus William entered into an agreement with Maurice Roe FitzGerald whereby he would marry Agnes and in turn she would receive joint-possession of his Limerick properties.
In this way, William received a local ally who would facilitate the maintenance of his Irish interests.
[2] Maurice, on the other hand, gained close access to the Crown and the opportunity for royal patronage and favour.