“Cult Killer” is a most peculiar Irish spree-killer thriller about the trauma of abuse, the monstrous sins of the super rich and some bizarre quirks of the Irish criminal justice system that one suspects are just clumsy inventions of the screenwriter.

But it stars the lovely and charismatic Alice Eve and the simmering Spanish screen icon Antonio Banderas and — you know — Ireland. So I’ll bite.

Banderas plays the guy no one in the pub should under-estimate, especially not the couple who think they’re sneaking around, cheating, when he’s been hired to follow them, bribe the barmaid to turn up the lights just long enough for him to snap a few shots and “serve” the offending faithless solicitor.

But Mikael sees something in the leather-pantsed blonde boozing, teasing and taunting three lugs in the corner. When they come after her as she knocks one last drink back and heads for the door, Mikael is a bit late in lending a hand. She’s a two-fisted drunk.

“Are you happy with a life that leads you to situations like that,” he wants to know? She isn’t. Yes, London-born librarian Cassie Holt has just found herself a sponsor, an ex-Interpol agent now working as a private detective, and a “friend of Bill.”

Five years later, she’s passed out on a bed, a lapsed alcoholic, when she gets the news. We’ve already seen Mikael chased into an alley and gutted by a disguised, knife-wielding woman.

Cassie Holt, who had taken on work as his researcher and assistant, is determined to catch his killer. As the Irish cop in charge (Paul Reid) sees this as another in a series of such murders which he’s having little luck solving, he’s happy to get help signs her in.

Say what now? A North American seeing how abruptly this under-trained aspiring private detective is brought into a police investigation will find this whole turn of events head-scratching. Charles Burnley’s screenplay wastes no time explaining how this might happen, and Reid’s Det. Inspector Rory McMahon just sort of makes it happen.

As the earlier victims of the nut-with-a-knife have been the super-secretive Old Money in the county, protected by snarling solicitor Victor Harrison (Matthew Tompkins), it’s quite jarring to see this green gumshoe accepted as “authority” when she starts flashing ID and asking uncomfortable questions.

At least we’re shown a lot of the mentoring Cassie got from Mikael, firearm and fight training in particular, in flashbacks. That’s enough to keep Banderas at the top of the bill if not satisfy the viewer’s puzzlement over the number of basic “rules” of the genre that Burnley and director Jon Keeyes break.

They give away the killer too soon, put her in girlfriend-to-girlfriend chats (split screens, each lying back on her bed) which reveal motive, the shared past trauma of these two and why the killer doesn’t kill “Cassie Holt,” whom she insists on calling by her full name — repeatedly.

Hitchcock said “Good villains make good thrillers,” but our killer isn’t “good” or painted wholly as a villain. Thank heavens the ferocious Olwen Fouéré, of the most recent “Texas Chainsaw Massacre” and Robert Eggers’ Viking saga “The Northman,” and screen veteran Nick Dunning (“The Tudors,” “The Iron Lady”) show up to give us some proper titled, amoral and monstrous bad guys.

Fouéré pretty much steals the picture, which despite its bizarre twists and taste of torture porn-level violence, takes on a tired familiarity common to such tales. From the private eye who breaks the law to help the cops catch a killer to the flashbacks that patiently explain how our gorgeous librarian obtained her “special skills,” “Cult Killer” can’t surprise its way out of a script that is basically tropes and trivialized trauma in a lovely Irish setting.

Rating: R, graphic violence, sexual situations

Cast: Alice Eve, Shelley Hennig, Paul Reid, Olwen Fouéré, Nick Dunning, Matthew Tompkins and Antonio Banderas

Credits: Directed by Jon Keeyes, scripted by Charles Burnley. A Saban Films release.

Running time: 1:45