An Irish citizen and nationalist, Liam once worked alongside Ian Doyle in the IRA. Their work relationship in a breakaway faction of the army (and later during their crimes in Valhalla and Lauren) seemed to imply that the two trusted each other extensively and that Liam may have been a lieutenant under Doyle's command. When Emily Prentiss, then working with Interpol's profiler organization JTF-12, infiltrated Doyle's faction under the guise of "Lauren Reynolds", an arms dealer, Liam became distrustful of her and even informed Doyle that he had to get rid of her. However, he was unaware that Doyle and Prentiss had started a romantic relationship in secret. It is unknown what happened to Liam when Doyle was first arrested and subsequently imprisoned at a North Korean political prison for the next seven years.
Liam was one of the few members of Doyle's faction who returned to support their leader upon his escape from prison in 2011. He and another member break into the Fagan residence and kill Frank and Kerry Fagan (the latter of whom was responsible for Doyle's incarceration) by tying them up and then asphyxiating both with a towel before covering up their deaths as a gas leak and subsequent house explosion. Liam then assists Doyle and two other members as they ambush Byron Delaney, another of Doyle's targets, and kill him. When confronted by Prentiss and Morgan, he and the other member shot at the two agents, engaging them in a shootout that left one member wounded. To prevent him from slowing the others down, Doyle shoots and kills the wounded member, with Liam watching. Later, he accompanies Doyle as he stalks Prentiss. Disapproving of Doyle's current tactic, Liam advises that it is easier to just grab her, citing that she will just get away again if they keep it up, but Doyle replies "No" and tells him to "have a little faith".
Liam later assists Doyle when he abducts and later tortures Prentiss. He also informs him that Jack Fahey has been arrested by the BAU, which prompts Doyle to give an order for a sniper to kill the former. When the BAU later deduced where Prentiss is being held captive, later breaching the building, Liam and the guards try to stop them, but they are all shot and killed by a SWAT team.
In Valhalla and Lauren, Liam usually operated with at least one partner, mostly Doyle. When he killed his known fatal victim, Kerry Fagan, he asphyxiated her with a towel and then covered up her death with the help of his accomplice. He later fired at Prentiss and Morgan with an automatic AKMSU carbine rifle at Byron Delaney's house.
No profile was specifically made of Liam, since the BAU were focused mostly on Doyle and, to same extent, the IRA faction that he led.
- Numerous unspecified crimes committed prior to Doyle's incarceration
- 2011, Washington, D.C., U.S.:
- March 3: Kerry Fagan (asphyxiated with a towel; covered her death up as a casualty of a gas leak and house explosion)
- March 4: The shootout at Byron Delaney's house:
- March 7: An unnamed SWAT agent (attempted; tried to shoot)