John James Brennan was as a doctor and worked in numbers locations in his career including Kilkenny city, England and with the Cunard line on the Mauritania Ship in the 1930s. During the War of Independence his clinic was overtaken by the Black and Tans and utilised as a base, forcing him to move his practice.
He married an American woman who worked for Eleanor Roosevelt as a secretary and they had one daughter. He was also a member of the Kilkenny hurling team, winning numerous All-Ireland medals c. 1904. He died in his mid 50s in Kilkenny.