Slicker Than Water: Bachleda and Farrell make a handsome seaborne couple.

Sea creatures, fairy tales, and Colin Farrell don’t seem a natural mix. Neil Jordan and unpretentious filmmaking aren’t exactly an expected pair, either. Which makes Ondine a lovely, low-key surprise. Farrell, continuing in effortless serious-actor mode after Crazy Heart and The Imaginarium of Doctor Parnassus, plays a greasy, working-class fisherman named Syracuse—but everyone calls him Circus, because he has a history of being little more than a drunken joke. He’s sober now but had to leave behind his wife and ill daughter to ditch the sauce. So all he has at the beginning of the film is a shack and his boat on the Irish Sea. (There’s not even a local AA chapter for support. For that, he visits his priest.)

Syracuse rarely catches anything, so it’s doubly surprising when he lifts his trawl one day and finds in it an unconscious—and gorgeous—woman (Farrell’s real-life squeeze, Alicja Bachleda). When she starts gulping air, they’re both surprised that she’s alive. She’s confused and skittish, insisting that she doesn’t want to go to the hospital and, in fact, doesn’t want to be seen by anyone besides him. Because he fished her out of the water, she says, but it probably doesn’t hurt that he’s handsome.

The woman eventually calls herself Ondine—“From the Waves”—and stays with Syracuse. The next time she’s on his boat, she sings a mournful, ethereal-sounding ballad in an unrecognizable language. (Jordan makes it even more otherworldly by echoing the audio and filming underwater.) And then Syracuse starts hauling in nearly more lobsters than his dinghy can hold. Which, combined with Ondine’s question mark of a past, lends credence to the theory of Syracuse’s beyond-her-years daughter, Annie (Alison Barry): That the woman is actually a selkie, a seal that can shed its skin and become human (and grant its host luck). In return for the booty, Syracuse buys/steals Ondine some nice clothes, including lingerie she’s frequently shown busting out of, and a quasiromance, unconventional-family thing starts.

For a film that focuses heavily on myths, wishes, and stories—Syracuse tells them to Annie and regularly asks her if anything “strange or wonderful” has happened, and Annie herself often talks about Alice’s Adventures in Wonderland as well as the legend of selkies—Ondine goes over like a gentle breeze. There’s darkness here, both literally, with scenes shot in ink-black night or on gray Irish days, and figuratively, with Annie’s mum still drinking and her boyfriend a fight-loving clod. Annie suffers from kidney failure and has to rely on a wheelchair, but it’s she more than Ondine who lends the film such lightness. Barry is terrific as the smart, curious kid who doesn’t feel sorry for herself; she spends her weekends “practicing my wheels” and insists her classmates are jealous. In turn, the adults in Annie’s life don’t condescend, with Ondine in particular forming an easy friendship with the girl.

There’s humor in the script, too, particularly the dry repartee between Syracuse and his priest (Jordan regular Stephen Rea) in the confessional. If the thick-brogued dialogue can be difficult to understand, little matter: Ondine is a sly piece of joy you won’t mind experiencing again.