Little is known about Kieran's background prior to 1899. His father was an Irishman with a dream of farming West in California, but inevitably died of cholera, while Kieran's mother suffered the same fate. This left a young Kieran orphaned and the stables where he worked threw him out shortly after. He joined the army but says it "didn't work out well". He then fell in with some unnamed outlaws, but they were killed by their line of work leaving him alone once again. At some point, Kieran encountered the O'Driscoll Boys, a rival of the Van der Linde gang. They said they would give him a choice, that he could "ride with them or die", which he said "wasn't much of a choice". He has a good knowledge of horses as he took care of them as a child, and again for the O'Driscolls. 
Events of Red Dead Redemption 2
As they head back from attacking the camp, they come across Kieran standing by the river, whom they recognize from earlier. Kieran bolts as soon as he sees the gang on horseback. Arthur captures him via lasso and takes him back to camp, and Dutch orders the gang to lock him up in one of the cabins and not give him food until he talks. Kieran tells them that he was part of the O'Driscoll gang unwillingly and begs to be let go, but to no avail.
Horseshoe Overlook Chapter
When the gang settles near Valentine, Kieran is seen tied to a tree, looking malnourished and weak. The gang decide to interrogate him again and threaten him; Kieran says he knows nothing. After the gang threatens to use gelding tongs on him (castrating him), he admits to them that he knows where one of Colm's supposed safe-houses is located. He leads Arthur, John and Bill to Six Point Cabin, the aforementioned supposed safe-house.
After Arthur and the other two clear out the camp of O'Driscoll members, Arthur heads for the cabin in the rear of the camp, assuming that Colm is hiding there on account of his body not being among those they killed. As Arthur opens the door, he is jumped by an O'Driscoll and is knocked to the ground. Before he can be killed, Kieran appears and fatally shoots the O’Driscoll, saving Arthur’s life.
When Arthur realizes that Colm O’Driscoll isn’t there, he assumes that Kieran set them up, and points his gun at Kieran. Kieran quickly argues that he would not have saved Arthur just moments earlier if that were the case, to which Arthur decides to reluctantly set him free, although Kieran instead insists he should join the Van der Linde gang, having made enemies of the O'Driscolls and would die in the wild otherwise. After some more convincing from John and Bill, Arthur eventually agrees and allows Kieran to stay with the gang.
Upon returning to camp an event may play where Bill teases Kieran about castrating him with gelding tongs again, which causes Kieran to run off.
Clemens Point Chapter
Kieran does not have much action during this chapter, he is usually just seen around camp performing chores during the day and sitting by himself at night, as he still doesn't feel as though the gang fully accepts him yet.
Kieran is not involved during any main quest and was the only male member of the gang besides Micah to be left at camp during the gang's attack on the Braithwaite Manor. Earlier on in the chapter he does however have a small mission where him and Arthur can go on a fishing trip together on a stretch of shore near the camp.
While fishing, the two men see another man skinny dipping in the water, and the man directs the two towards another tip of shore that had some abnormally large fish. Kieran and Arthur head in to the new spot pointed out, where Arthur can catch the Legendary Bluegill. After Arthur's first few attempts Kieran will say it is getting late and ride back to camp, leaving Arthur to fish a little longer.
Saint Denis Chapter
At some point after the gang relocated into Shady Belle, Kieran was captured by the O'Driscolls. He was tortured until they found out the Van der Linde gang's location. After he had told them everything he was decapitated, his eyes gouged out, and his corpse was sent to Shady Belle on horseback. His corpse was tied up in a manner that displayed his decapitated head being held in his cupped hands. Soon after, O'Driscoll Boys ambushed the Shady Belle manor.
Following the gunfight, Dutch and the rest of the gang began to clean up the mess. Dutch ordered Reverend Swanson to bury Kieran somewhere that's "close, but not too close".
Arthur later wrote about his regret at being unable to save him, even though he saved him, in his journal. Kieran's death also had an impact on the gang's already deteriorating emotional state, most notably Arthur, Bill and Mary-Beth.
Beaver Hollow Chapter
Colm O'Driscoll was later captured and sentenced to be hanged. Arthur, Dutch and Sadie attended the execution in disguise to ensure the sentence was carried out; Dutch pointed out Kieran's fate as one of the reasons why they had to succeed. Colm was hanged in the end, avenging Kieran's death.
Kieran is described by Arthur as a "gentle buffoon". He is soft-spoken and nervous, often stuttering when he speaks. He generally has a kind-hearted demeanor and avoids conflict when he can, for example agreeing to drink with Bill Williamson even after Bill physically threatened him so often. When he tells Sean MacGuire his life story of being orphaned and falling in with the O'Driscolls, he says that all he's ever known is "folks all seem to die", reflecting that he may be wary of losing friends and loved ones so often and never having a solid home or family. Although the gang kidnapped him, he seems intent on earning their trust and becoming a fully fledged member. He tells Arthur that he plans on proving himself to him. When Jack is kidnapped he tells Abigail that he would give his life for Jack if he could. He becomes genuinely frustrated whenever someone refers to him as an "O'Driscoll", and told Dutch upon meeting him that he hates Colm O'Driscoll.
Kieran has green eyes and dark, shoulder length hair. He's unshaven. When Karen first sees him tied to the tree, she says she "thought O'Driscolls' ate babies, but [he] looks like a baby."
- "Old Friends"
- "Eastward Bound"
- "Paying a Social Call"
- "The New South"
- "Blood Feuds, Ancient and Modern"
- "The Battle of Shady Belle"
- "Horsemen, Apocalypses" (Corpse)
- Kieran appears to have a crush on Mary-Beth and vice versa. Mary Beth is first seen giving him water while he is tied to the tree in camp, an act which appears to be the only show of kindness Kieran receives whilst tied up. Later Kieran can be seen sitting by her in camp where he tells her she's "very pretty". A third encounter can be seen between the two where Mary Beth inquires if Kieran is able to read or not. When Kieran says he never learnt, Mary Beth flirtatiously remarks that "maybe someone can teach [him]" (alluding to herself), which causes Kieran to get visibly flustered and leave. When Kieran is missing, Mary Beth can be seen frantically asking if anyone has seen him or not, and remarks that she's worried about him. After his death Mary Beth will say that she "always had a soft spot for him".
- Based on dialogue in the mission "No, No and Thrice, No", Susan Grimshaw mentions that she hadn't seen Kieran around the camp recently, alluding to his eventual demise.
- Bill Williamson is heavily impacted by Kieran's death. Although Bill initially seemed to take joy from terrifying and threatening him, he can later be seen in camp awkwardly trying to encourage Kieran to sit and drink with him, and he gets upset when Kieran runs off scared. Bill likely developed a liking for Kieran in the end and felt guilty for how the gang initially treated him.
- His grave site can be found near Bolger Glade. This can be visited after "Horsemen, Apocalypses".
- He appears to have a Cattleman Revolver which has an iron frame and a bone grip.
- In the mission “Paying a Social Call”, Bill tells Kieran "You'll die, boy, but you‘ll lose your balls first". Kieran’s eventual fate of suffering and then dying is foreshadowed by Bill to some extent, and, given that his eyes appear to get gouged out, it appears his fate was foreshadowed fairly literally.
- Kieran's death is more likely a reference to the Catholic martyr, Saint Denis, who was decapitated and is often depicted in art holding his head in his hands. This is further confirmed since the featured city in the chapter is also named Saint Denis.
- During the third conversation he has with Mary-Beth, it appears that Kieran is illiterate.