He was born at Swords, County Dublin. The FitzRerys were a long-established family of Welsh origin who came to Ireland  before the Anglo-Norman invasion of Ireland and settled at Cloghran in Swords, where they remained substantial landowners until the seventeenth century. Their claim to be descended from Gruffudd ap Cynan, King of Gwynedd (1055-1137), is not implausible since Gruffudd was born in Swords to an Irish mother, had numerous non-marital children, and retained close links with Dublin in later life.