Answer: Tony Gardello
We can see the boxer's name, in the newspaper article during the flashback scene. Sean says his name, while speaking with the Reverend. Sean tells the Reverend Playfair that he didn't just go into the ring to beat him, he went in to "beat his brains out". The Reverend asks Sean if his wife's love is worth fighting for. Sean tells him that he doesn't know. "I don't know. About all I know is I can't fight or won't fight unless I'm mad enough to kill, and if that means losin' her, I don't know. Maybe she doesn't love me enough." His death is what made Sean quit boxing and return to Ireland. He was tired of fighting for purse money and he was haunted by the boxer's death. Did you notice the large book, presumably a Bible, which moves magically from in front of the fire place to the window?