Per his military service record (Depot), John Brennan was born at Brisbane. He gave his age as 32 years and 2 months, his marital status as single, and his occupation as dealer. His description on his medical was height 5 feet 4 inches tall, weight 132 lbs., with a dark complexion, brown eyes, and black & grey hair. His religious denomination was Roman Catholic. He claimed that he had no previous military service.
John Brennan completed his medical examination on 26th October 1915 at Wellington (8 days after the Coo-ees had left through that town), then travelled to Blayney to catch up with the Coo-ees. He was attested at Blayney by Captain Eade on 26th October 1915. He claimed that he had no previous military service.
His address was recorded as ‘P.O. Geurie’ on his initial Application to Enlist in the Australian Imperial Force form .
It appears that John Brennan may have been one of the four unnamed men reported in the Wellington Times who signified their intention of joining the Coo-ee March at the appeal made at the Church of England Ball the night the Coo-ees stayed at Geurie on 15th October 1915.
The Farmer and Settler reported that five men arrived to join the Coo-ees at Blayney on the 26th October 1915, by the late train from Geurie. The Dubbo Liberal and Macquarie Advocate named ‘J. Brennan’ as one of four recruits from Geurie that caught the evening mail train at Wellington to join the Coo-ees at Blayney, (joining A. Lynne [known as a Gilgandra recruit] and W. Smith, who had caught the train at Geurie).
After completing the march he went to Liverpool Camp as reinforcement for the 13th Battalion.
On 17th November 1915 Private Brennan went before a Medical Board at Liverpool Camp, where he was diagnosed to suffer from varicocele, and unfit for active service.
On 4th December 1915 Private Brennan was discharged from the A.I.F. as medically unfit.