Gaffney is a Manhattanassistant district attorney, consistently working alongside Tracey Kibre (Bebe Neuwirth). Of the two ADAs, Gaffney is the younger assistant who comes from an Irish Catholic family with a history that runs deep in the City of New York. Gaffney is a graduate of Boston College, and is often more by-the-book when it came to law and its application than her superior. She often challenges Kibre in pertinence to cases they prosecute, and sees the law in more shades of gray than her partner.
Gaffney always sits second-chair to Kibre while prosecuting. Her role during trials is limited, as Kibre often tries the entirety of most cases the pair prosecute. Gaffney's role is often most significantly played out at the crime scene investigation at the start of the show, at which point she often goes to the crime scene and interviews those who either witnessed the crime around which the show is set to revolve or were in the vicinity of the crime scene during its occurrence. The two prosecutors usually try homicides, though they try a high-profile rape during a crossover with Law & Order: Special Victims Unit when ADA Casey Novak (Diane Neal) is viciously attacked.
Gaffney is more idealistic than Kibre. In one episode, for example, a jury returns a verdict of guilty in a case in which a babysitter is charged with murder due to Shaken Baby Syndrome. The judge, however, overrides the ruling, at which point Gaffney suggests to Kibre that they need to do something, whether it be filing an appeal or having the judge transferred to Staten IslandTraffic Court. Kibre tells Gaffney that all they can do is move on.