Dillon had been studying law in London at
Gray's Inn and was called to the
bar in 1613. He was knighted on 15 February 1615 by
James I at
Theobalds House, Hertfordshire. Dillon's parents as well as his first wife were Catholics. His eldest son,
James, was originally raised as a Catholic. Dillon as well as his son James conformed to the established religion some time before his father's ennoblement. His father, however, remained Catholic. On 24 January 1620 Dillon's father was raised to the peerage with the title "Baron Dillon of Kilkenny-West", in the
Peerage of Ireland in a ceremony performed by the
Lord Deputy Oliver St. John in the presence chamber of
Dublin Castle on 25 January. On 5 August 1622 Dillon's father was advanced to Earl of Roscommon. This made the title "Baron Dillon of Kilkenny-West" available as
courtesy title for the heir apparent. Dillon, therefore was 1622–1641 styled "Robert Lord Dillon of Kilkenny-West" or "Robert Lord Dillon" fort short. == Third marriage ==