"Do you know who I am? I am the man!"
The details of Fahey's past are not elaborated on. Several years ago, he was the one who introduced Prentiss, whom he only knew by her undercover name, "Lauren Reynolds", to Ian Doyle, a former member of the IRA who became an international arms dealer. Fahey was apparently never privy to Prentiss's true identity. At the time of the introduction, which took place in 2003, eight years prior to "Lauren", Fahey was apparently respected in his neighborhood, likely due to belonging to the local Irish mob, but eventually fell on harsher times.
At the beginning of "Lauren", Prentiss seeks out Fahey at the bar where she first met Doyle. When she sees him there, she sneaks into his car and holds him at gunpoint and forces him to reveal the location of Doyle's hideout. Afterwards, she shoots him through the right ear as a warning. The BAU track Fahey down through his frequent phone calls to Clyde Easter, who promised to pay his medical bills in exchange for information, and arrest him.
While being interrogated by Rossi and Reid, Fahey learns how close they are to Prentiss and demands $200,000 in exchange for any more help. In an attempt to get Fahey to relax and open up, Rossi and Seaver take him to the building's balcony so he can have a smoke. While they are talking, one of Doyle's henchmen oversees the meeting from another building and prepares to snipe the three of them. Doyle initially wants the sniper to kill first Seaver, then Rossi, and finally Fahey, if possible; if the shot fails, he figures he will probably be too scared to talk anyway. However, Prentiss convinces Doyle to have the sniper only kill Fahey and let Rossi and Seaver live. Doyle agrees and has the sniper shoot Fahey twice in the chest, killing him almost instantly.