Sanity Slippage: Working a murder case near the scene of his friends' unsolved disappearance has an alarming effect on his psyche.
Smart People Speak the Queen's English: Although natively Irish, he picked up a British accent while attending Boarding School in England and notes that the presence of this trope in the minds of the public has probably led those around him to believe he's smarter than he actually is.
Trauma-Induced Amnesia: Most of the first twelve years of his life are a blank. Throughout In the Woods he recovers his memories but remains unable to remember the circumstances surrounding Peter and Jamie's disappearance.
Becoming the Mask: A risk inherent in undercover work, one which she was well able to avoid during her time infiltrating a drug ring; by The Likeness, though, recent trauma has primed her for Sanity Slippage.
Dark and Troubled Past: As a college student, she befriended a psychopath who turned her friends against her when she refused his advances, claiming that he had turned her down and that she had threatened to falsely report him for rape. This experience directly lead to her decision to become a detective.
Dark and Troubled Past: Was raised in an abusive home and planned to run away with Rosie, the love of his life, but then she seemingly ditched him the night they were supposed to leave, and he ended up homeless for a while before finally becoming a cop. Then it turns out Rosie didn't dump him, she was murdered, and has been dead in the basement of the old house they used to hang out in for eighteen years. Oh, and his brother did it.
Knight Templar Parent: Nowhere more evidently than in The Secret Place; if Stephen Moran is to be believed, he'd knowingly have an innocent teenager convicted for murder in Holly's place without a second thought, even if he knew Holly to be the real killer.
Overprotective Dad: He knows very well what bad things are in the world, and he's going to keep his daughter away from them.
Papa Wolf: When he first laid eyes on his newborn daughter, he was filled with the urge to kill someone so that she'd never have to question what he'd be willing to do for her. Time does not lessen these emotions.
Family Versus Career: His wife left him after deciding that she wanted children, which he refused to have, claiming that murder detectives who become parents inevitably are unable to cope with cases that involve children. The truth is more complicated; he's afraid of being a bad parent.
Heterosexual Life-Partners: Although he prefers to train rookies than to work with a partner, he sees himself starting to develop this sort of relationship with newbie detective Richie Curran in Broken Harbour.Unfortunately, it's over before it begins.
Good Cop/Bad Cop: Usually plays the good cop to Conway's "scary bitch" interview persona.
Nice Guy: Probably the least troubled of the protagonists.
Obfuscating Stupidity: His inner-city Dublin accent often leads people to assume he's stupid, and he's happy to let them.
Protagonist: The Trespasser
Ambiguously Brown: Due to her father being a Disappeared Dad and her mother being a chronic liar about his identity. She's probably half-Arab, but it's uncertain, and she doesn't care about her father enough to actually hunt him down and ask.
Good Cop/Bad Cop: Usually bad cop in any equation involving her partner, but will play nice on her own or with another detective.
Jerk with a Heart of Gold: She's abrasive and aggressive, but this is mainly because of constant harassment from some old boys' club types, and she really does care deep down. (After the ringleader is finally fired, it's implied she'll mellow out a little.)
The Lad-ette: Not actually, but this is one of her interview personas, the "cool girl" who's like one of the guys. She finds it gets male interview subjects to open up to her.
Screw This, I'm Outta Here!: By her own book, the racist/sexist harassment in Murder has escalated to the point she's considering quitting the force and taking a job as a bodyguard for rich women. (She almost does it, except 1) she realizes Stephen has been backing her the whole way, and 2) it turns out there was only one major offender, not almost everyone else like she thought.)
Twofer Token Minority: Invoked; her nastier coworkers joke behind her back (but not far enough behind that she can't hear them), that she only got onto the squad because she fills multiple quotas as a biracial woman.
You, Get Me Coffee: In one of her first interactions with Stephen Moran in The Secret Place, she sends him into a coffee shop. He's fine with this, less so when she dislikes it and asks him to throw it out five seconds later. (This is a Secret Test of Character, as, given the behavior of some of her male coworkers, she wants to know if he'll balk at a woman giving him orders before anything important happens.)