He attended High School at St. Augustine Academy and played basketball there for four years.
[3] When he graduated in 1919, he went directly to a professional basketball team: the all-Irish Brooklyn Visitations.
This at times prevented him from playing in road games when he was an active basketball player during the 1920s and 30s.
During Brennan's seven seasons the Terriers, playing in the Metropolitan New York Conference or as Independents, had a 90–46 record.
Brennan has the highest win percentage as a head coach in Terrier basketball history at 66.2%.