[2][3] His father was the eldest son of his grandfather James Dillon, nicknamed the Prior,[4] because he took care of several monastic properties at the dissolution of the monasteries.

[38] During this period of the English reconquest in Connacht, Dillon was involved in some sharp practices with the local landholders.

In particular, he persuaded the various Costello freeholders of the Barony of Costello in eastern County Mayo, to save expense and ensure the smooth legal transfer, to allow him to surrender their lands for them in one land-title in the Surrender and regrant process and had it regranted in his own name, becoming the legal landowner in the process.

He never returned this title to the lands to the native owners, which would lead to rapparee actions by Dudley Costello (or Costellogh) against the Dillons in the 1660s.

[42] On 19 July 1608, King James I gave him a patent confirming the possession of the manor and town of Kilfaughny in County Westmeath[43] where he then lived and finally died.

On 16 March 1622, King James I created him Viscount Dillon of Costello-Gallen, cementing his legal title.