Claire Keegan’s novella “Foster” follows last year’s beautiful, heart-wrenching “Small Things Like These,” which went on to be short-listed for the Booker Prize. A short novel of uncommon spareness and delicacy, it tells the story of a man faced with a choice that pits his conscience against the welfare of his family.

“Foster,” which is as finely wrought and subtle as that novel, is slightly revised from its first appearance in the New Yorker in 2010. It is also the basis for the Irish-language film “An Cailin Ciuin” (“The Quiet Girl”) released earlier this year in Europe, where it proceeded to collect numerous prizes and high accolades.

At the story’s center is a young, unnamed girl who has been brought up in a wretchedly poor farming family with too many children on the southeast coast of Ireland. Her father, a feckless gambler, drinker and habitual liar, neglects the farm, treating his responsibilities with insouciance and his daughter with casual spite; her mother, pregnant yet again, is ground down by childbearing and ceaseless drudgery. Kindness is a stranger to this home and a passing mention of the car seat that serves as the family’s sofa is eloquent of their squalid way of life.

The girl has been taken by her loutish father to spend the summer with relatives, a married couple whom the girl does not know. They too are farmers, but diligent and conscientious, their place well regulated and prosperous. It’s strange and intimidating to this raw young person, but both man and wife are kind and the girl gradually begins to shed her feeling of awkwardness and shame at her own crude ignorance.

Watchful and attentive, she begins to relax, her growing confidence and blossoming personality shown in immensely subtle ways, as when she gets a joke and laughs — probably for the first time in her life. But there is secret sadness here, too. A malicious gossip tells her how the couple lost a son around her age, and it is clear that she is beginning to take the boy’s place in the mother’s heart.

At the same time, as the girl’s love for this couple grows, an air of menace gathers. Another secret materializes. And always, over the reader, if not the girl, the question hovers: Will this happy, vitalizing passage in her life end? Will she be returned to her bleak, loveless home?

There are in this story, as in all of Claire Keegan’s work, layers of nuance and resonance. Every detail has bearing, some quietly salient, others possessing a delayed charge, so that, again and again, the reader feels the sharp thrill of comprehension. A stray heifer, a light at sea, that joke understood, all pregnant tokens of a fully realized universe of feeling. Keegan is the finest writer at work in Ireland today and this brilliant little book is further proof of it.