Colm O'Driscoll was a long-time infamous outlaw operating across the Old West. In his earlier days, he had a sort of partnership with Dutch van der Linde, a fellow infamous outlaw. The two would go on to found gangs named after them, the O'Driscoll Boys and the Van der Linde gang respectively.
Their odd friendship eventually turned into bitter enmity when Dutch killed Colm's brother. In retaliation, Colm murdered a woman called Annabelle, implied to have been a lover of Dutch. Ever since these events, Colm and Dutch's gangs were involved in a blood feud.
Over the following years, Colm would expand his operations considerably, turning his gang into one of the largest and most powerful criminal organisations in the Old West. He was arrested and convicted for murder twice, but managed to escape on both occasions before he could be hanged.
Events of Red Dead Redemption 2
Colm is discovered to be planning a robbery of a train owned by Leviticus Cornwall. He is seen bullying Kieran in the O'Driscoll camp in the mountains. He leaves the camp shortly before the Van der Linde gang wipes it out and robs the train for themselves.
Clemens Point chapter
Pearson comes to Dutch and informs him that the O'Driscoll gang is amenable to a truce and that Colm has decided to meet with Dutch to parley, but this turns out to be a ploy to capture Arthur Morgan. Colm later confronts the imprisoned Arthur, explaining that he made a deal with the Pinkertons. He is counting on Dutch to launch a rescue mission, upon which the trap will be sprung and the Van der Linde gang arrested. His plans, however, are foiled when Morgan manages to escape.
Beaver Hollow chapter
Colm is arrested by the authorities and is sentenced to be hanged in Saint Denis. Dutch, eager to watch him die, personally attends Colm's execution disguised as a police officer, while bringing Arthur and Sadie Adler with him. The three quietly ambush the O'Driscolls present to rescue Colm. Colm, initially cocky that his gang would save him, upon realizing his escape plan had been ruined, began panicking before he was finally hanged.