James was born in Ireland, the eldest son of Lucas Dillon and his wife Jane Bathe. His father was a judge and Chief Baron of the Irish Exchequer. His family was Old English and descended from Sir Henry Dillon who came to Ireland with Prince John in 1185. His family held substantial lands in Meath, Westmeath, Longford and Roscommon. James's mother was a daughter of James Bathe, who also was a Chief Baron of the Irish Exchequer. She was his father's first wife. His father's second marriage seems to have been childless.
James was one of seven brothers and had five sisters.
Stepmother
His mother, Jane Bathe, must have died about 1570 as his father remarried in 1575 to Marion Barnewall, née Sharl, the widow of Sir Christopher Barnewall of Turvey House, Dublin. Marion, his stepmother, had 15 children from her first marriage, among them Eleanor with whom James fell in love.
Marriage and children
James married Eleanor Barnewall, also called Helen, his step-sister through his father's second marriage. She was a daughter of Sir Christopher Barnewall. James and Eleanor had seven sons:
His father died in 1592 in Dublin. There must have been some complications with the inheritance as he obtained special livery of his inheritance on 8 April 1595. In 1599 Dillon raised a troop of 25 horse loyal to Elizabeth I at his own expense, to help keep order in County Roscommon. His grandson James, his son Robert's the eldest son and future 3rd Earl, was born in 1605. Dillon was knighted by King James. On 24 January 1620 Sir James Dillon, as he was now, was raised to the peerage with the title of Baron Dillon of Kilkenny-West, in the Peerage of Ireland. This elevation was announced in a ceremony performed by the chief governor of Ireland, Lord Deputy Oliver St. John, in the Presence Chamber of Dublin Castle on 25 January. On 5 August 1622 Lord Kilkenny-West was advanced to the dignity of Earl of Roscommon. His baronial title became a subsidiary title, which he gave as a courtesy title to his heir apparent as is the custom. His eldest son Robert therefore was styled Lord Kilkenny-West 1622–1641. Lord Roscommon was a signatory of a response to Charles I from the Lords of the Pale that established a military force to protect The Crown's interests in Ireland. In 1627, he was a Commissioner for raising money for the King's Army in Meath, Westmeath and Longford. His wife predeceased him on 11 October 1628. On 14 July 1634, Lord Roscommon took his seat in the Irish House of Lords. This was the first Irish Parliament called by King Charles I. He died in March 1641 and was succeeded in his titles by his eldest son, Robert as the 2nd Earl of Roscommon.