From his IBHOF biography:
"Born November 26, 1873 in Dundalk, Ireland. As a youth, he ran away from home and went to sea as a cabin boy. Sharkey arrived in New York in 1892 where he joined the U.S. Navy. Quickly promoted to "master-at-arms" he was deployed to Hawaii where he began his pro career with 20 consecutive knockouts.
In 1896 he met his first stern challenge when he went the distance with Hall of Famer Joe Choynski and claimed victory after 8 rough rounds. World heavyweight champion James J. Corbett witnessed the bout and challenged Sharkey, confident his boxing ability would be too much for the young Irishmen. The 4-round bout was declared a draw due to police interference, but spectators had no doubt that Sharkey had bested the champion. Sharkey next engaged his idol, John L. Sullivan, in a 3-round exhibition bout before defeating future heavyweight king Bob Fitzsimmons via a controversial 8th round foul (Referee Wyatt Earp ruled that a right hand that felled Sharkey for the 10-count was a low blow). In 1898 he lost a close decision to future champion James J. Jeffries, topped Gus Ruhlin in one round and defeated Corbett via 9th round foul.
A 10th round stoppage over Kid McCoy set up an 1899 world heavyweight title contest with Jeffries at Coney Island. After 25 furious rounds, Jeffries was declared the winner by referee George Siler in a disputed decision. Sharkey continued to box, stopping Joe Goddard and Choynski before losing back to back bouts to Ruhlin and Fitzsimmons. He retired from the ring in 1904 with a record of 40-6-3ND (37KOs).
Although never a champion, the barrel-chested slugger is regarded as a ring immortal of his era. In retirement he toured with Jeffries in vaudeville shows. He died on April 17, 1953 in San Francisco."