[4] Though many sources of his time stated he was born in County Mayo, Ireland or Chicago, these are fabrications he perpetuated to bolster the myth of his Irish ancestry.

According to one source, Brennan achieved a third-round knockout of Jack Cameron in 1914, and two ten-round newspaper decisions from Marty Cutler on April 13 and July 2, 1914, in Aurora, Illinois.

On March 20, 1916, Brennan knocked out Brooklyn-born Italian boxer Al Benedict in the second round at the Military Athletic Club in Brooklyn, New York.

On May 1, 1916, Brennan achieved a second-round knockout of 6 foot, Buffalo native George "One Round" Davis, a competent heavyweight, in Rochester, New York.

[1] On May 16, 1916, Brennan achieved a sixth-round technical knockout of Al Williams at the Pioneer Sporting Club in New York City.

[1] On December 30, 1916, Brennan defeated Jack Keating in a third round Technical Knockout at Queensboro Athletic Club in Long Island City, in Queens, New York.

Though not having a stellar record, Carroll met some top talent during his career, including Gunboat Smith and two bouts with Battling Levinsky.

Carroll lost to Levinsky on March 9, 1916, in New London, Connecticut, in a match promoted by Jewish boxer Abe Hollandersky.

[1] On June 12, and July 20, 1917, Brennan achieved two tenth-round newspaper wins at first the Broadway and then the Harlem Sports Clubs against Bob Devere in New York City.

[1] On November 1, 1917, Brennan first defeated Joe Bonds at the Unity Cycle Club in Lawrence, Massachusetts, in a second-round knockout.

[1] On November 9, 1917, Brennan drew with 6' 3" New York heavyweight Jim Coffey by newspaper decision in ten rounds at the Harlem Sports Club.

[1] On December 1, 1919, Brennan knocked out five foot eleven inch Irish boxer Tom "Bearcat" McMahon in the second of fifteen rounds at Heinemann Park in New Orleans.

[1] One of his toughest and best known opponents, Brennan fought Harry Greb four times in 1919, losing three bouts by newspaper decisions, and one on points.

On July 4, 1919, Greb met Brennan at Convention Hall in Tulsa, Oklahoma, and won the bout in a fifteen-round points decision.

[12] Brennan fought future Hall of Famer Billy Miske twice in 1919, once on June 28 in Oklahoma to a fifteen-round loss on points, and on April 28 at Cardinal Field in St. Louis to an eight-round newspaper decision draw.

Brennan first lost to Jack Dempsey in a non-title fight on February 25, 1918, in Milwaukee, Wisconsin, in a sixth-round technical knockout.

[2][13][4] Appearing to be in better condition than he was in their first meeting, for the first nine rounds of the Dempsey title fight on December 14, 1920, in boxing's greatest venue, Madison Square Garden, Brennan held his own against the Champion.

Dempsey was never one to miss an opportunity to finish an opponent, and the left hook that put Brennan down for the count was the best punch in his arsenal.

The St. Louis Post-Dispatch wrote that Dempsey "was forced to his utmost to hold off a courageous, determined fighter who gave almost as good as he received."

Dempsey's most noticeable injuries were a split left ear and a lighter blow to the mouth which both bled at times during the bout.

Famed sportswriter Grantland Rice wrote that Dempsey's ear looked like "a cross between a veal cutlet and a bloodly sponge", as Brennan had targeted it in the fight.

and it occurred before a sellout crowd of nearly 17,000[2][4] On February 18, 1921, Brennan defeated Bob Martin in a fifteen-round points decision at Madison Square Garden.

[19] On March 31, 1921, a few sources have Brennan losing to talented Black boxer Jeff Clark in Atlantic City, New Jersey.

[21] Brennan fought Luis Ángel Firpo only once, on March 12, 1923, at Madison Square Garden, losing badly in an important twelfth-round knockout.

[22] Brennan was not released from the hospital till around March 21, and it was intimated at the time that he or his handlers were considering his permanent retirement from the ring.

After Brennan opened a bar, Club Tia Juana, on 171st Street in the Washington Heights section of Manhattan, some rough looking men informed him that he was buying the wrong mob's beer.

Three other men were present near the gunman, and suspected of being involved in the incident and also having connections to the "Hell's Kitchen" gang which had targeted Brennan.

[6][16][27][28] On June 18, after 5,000 attended his Memorial service at New York's St. Rose de Lima Catholic Church, his funeral was held.