|Dates||1985 - 2017 (killed by a hellhound)|
|Episode(s)|| 11.11 Into the Mystic|
12.17 The British Invasion
12.21 There's Something About Mary
I prefer working alone. Trust issues.
– Eileen Leahy, 11.11 Into the Mystic
Eileen Leahy is an Irish-born hunter. Thirty years ago her father was killed by a banshee and her mother sacrificed herself to protect Eileen by performing a spell that sent the banshee away. A hunter named Lillian O'Grady found the infant Eileen in her crib with her ears bleeding, having gone deaf from the banshee's piercing scream. Lillian raised and trained Eileen to be a hunter, while also attempting to track down the banshee that killed Eileen's parents. Lillian died of cancer when Eileen was 16 years old, leaving her on her own to hunt. Eileen has continued to try to find the banshee that killed her family and get revenge.
Having learned of a recent banshee kill, Eileen goes undercover as a housekeeper at Oak Park Retirement Living in Oak Park, Kansas. There she crosses paths with Sam Winchester. Misreading his and Dean's conversation, she assumes Sam to be a banshee that has learned to blend in. She traps Sam with a Celtic sigil trap in the laundry room. After she cuts him with a gold blade that a banshee would react to, Sam is able to convince her that he and Dean are both hunters, looking for the same banshee that she is.
The Winchesters get together with Eileen and Mildred, whom they suspect may be the next victim. Eileen and Mildred discuss how hot the boys are using American Sign Language.
Together, the three formulate a plan to trap and kill the creature, using Mildred Baker as bait, in a room that has been adorned with a Celtic sigil trap. The banshee appears and goes after Dean. When Eileen attempts to kill it, she is flung across the room into some shelves. After Sam is incapacitated as well, Eileen signs to Mildred to activate the sigil, which sends the banshee into a wall. Eileen quickly grabs a gold blade and rushes the trapped banshee, stabbing it in the midsection, causing it to explode into black smoke and dissipate.
Once the case is finished, Sam asks Eileen how her revenge felt and what she plans to do now. She tells him it was just another kill and that hunting is her life. He tells Eileen to call if she ever needs anything, or even to just hang out. Eileen and Mildred admire the view as Sam and Dean walk away from them towards the Impala.
Speaking with Sam and Dean through FaceTime, Eileen tells them she has been tracking Kelly Kline's whereabouts. She reveals that Kelly's last known location in Coeur d'Alene, Idaho, was a warehouse that mysteriously burned down, and she sends them security photos of Kelly leaving the building with Dagon. Dean tells Eileen she's doing great work and gives Eileen the thumbs up as Sam shifts the monitor back towards himself and the two say their goodbyes.
Some time later, Eileen meets up with Sam and Dean at the Bunker. She tells them she ran the plates on any cars that passed the warehouse that night, and reveals the name Dermott Culp, a man that went missing a year ago. Tracking his car to Iowa, she'd found Dermott carrying a dead body out of a building. When Dean questions if Dermott is a killer, Eileen tells him he was a demon who cleaned up Dagon's messes. Upon confronting him, the two had quickly gotten into a fight where Eileen was able to subdue him, and before stabbing him in the heart with an angel blade was able to get Kelly Kline's phone number from him, much to Sam and Dean's delight.
The next day at a junkyard, Sam and Eileen meet up with Mick Davies and Renny Rawlings, who hand over the Colt to Sam. Later that night, when Dean arrives with Kelly, Mick argues that Kelly will mean nothing to the Nephilim, when suddenly Dagon appears. Dagon thrusts her hands in the air and Sam, Dean, Eileen, Mick, and Renny get flung backwards, landing heavily. As they slowly get back on their feet, Dean and Mick try shooting Dagon, which has no effect in slowing her down. She throws them aside again and walks up to Kelly. Eileen sees the Colt, which Sam dropped when he was thrown. She inches towards it. Dagon reaches Kelly, takes her by the hand, and begins to walk away. With Dagon in her sights, Eileen fires, just as Dagon disappears, taking Kelly with her. The bullet from the Colt continues on its path and hits Renny in the chest. Sam, Dean, Mick, and Eileen slowly realize what has happened as Renny collapses and dies. Shocked, Eileen tells everyone she didn't mean to kill Renny, and was aiming for Dagon. Mick pulls out his gun and approaches Eileen, telling Sam and Dean that since she killed a Man of Letters, she must die. Eileen pleads with Mick not to kill her, and Mick is eventually talked down by Sam and lets the three go.
At the Bunker, Dean asks if Eileen is okay. She nods with a yes, before shaking her head and breaking into tears, telling the Winchesters that Renny wasn't a monster. Eileen reaches for Sam and he pulls her into a comforting hug, telling her it was a mistake. Sometime later Dean questions Sam where Eileen is at; Sam tells him that she went back to Ireland for a while.
As Mick is giving his report to Dr. Hess, she tells Mick that Eileen Leahy will be found and killed in accordance with the Code.
Eileen is running for her life through some woods in South Carolina, being pursued by a hellhound. She stops and begins firing her weapon but is quickly overtaken and mauled, until Mr. Ketch blows a dog whistle, calling the hound off of Eileen's now dead body.
Sometime later Sam receives a call from Jody informing him of Eileen's death. He and Dean head down to South Carolina to identify her body and come to the conclusion they are dealing with a hellhound. While investigating the recent rash of hunter deaths, Sam and Dean stop at the Lebanon, Kansas, post office to check their P.O. Box, where they find a letter from Eileen, telling them she'd found a bug in her room and believed her phone and computer were hacked, and asking to stay with Sam and Dean at the Bunker for a few days.