The St. Louis Union Club have signed young Brennan, who caught Hodnet so well in Sunday's game. Brennan is the youngest player on the Union team, and one of the most promising. He caught last year on the local Grand Avenue and Liberty teams, Werden being his pitcher. He also went to Chillicothe, Mo., with Werden, and played quite a good game up there.-St. Louis Globe-Democrat, April 8, 1884
Jack Brennan was actually born John Gottlieb Dorn. I have no idea why he changed his name unless it was because he wanted to hide his professional baseball activities.
Brennan appears to have played an important role on the Maroons as a utility guy and first guy off the bench. He played in 56 games and appeared at catcher, third base, short and in the outfield. He went on to have about as successful career as a utility guy who can't hit can have, playing five seasons in the majors and continuing his professional career into his thirties.
There's a couple of good Jack Brennan stories here and here, if you're interested.