Like William Marshall, John was linked to the court of Henry II. His father was the king’s chamberlain but he died when John was still only 8. D’Earley senior left money towards the foundation of Buckland Abbey in Devon and John became a royal ward. In 1185 or thereabouts when John was 14 his wardship was passed into the hands of William Marshal. In time John became Marshal’s squire and married Sybil who was probably Marshal’s illegitimate niece. In 1194 John was knighted by Marshal and took control of his estates.

Where Marshal went so did John. And when Marshal was summoned back to England in 1207 because King John was feeling spiteful it was John who remained with Marshal’s pregnant wife. He was responsible for the administration of the southern half go Leinster. When King John’s plans to ruin Marshal failed to work D’Earley was one of the knights summoned to England in 1208. He was allowed back to Ireland but was taken hostage by King John when Marshal protected William De Braose. John found himself in Nottingham Castle for a time. But when the king needed Marshal D’Earely was released and was also given custody of Marshal’s eldest son William. By 1213 D’Earley was marshal of the royal household and Sheriff of Devon.

D’Earley was by Marshal’s side during the First Baron’s War, for Magna Carta and for the regency of King Henry III. When Marshal fell ill and was transported by boat to Caversham in 1219 D’Earley went to Marshal’s side and remained there until his mentor and friend died. He continued to be part of the Marshal affinity until his own death. England’s most loyal knight had a very loyal knight of his own.

https://www.berkshirehistory.com/bios/jdearley.html