The late film critic Roger Ebert once wrote, "What moves me emotionally is more often goodness than sadness." It's a sentiment I've always shared, and I thought about it again while watching the beautifully crafted Irish drama The Quiet Girl.

There's plenty of sadness in this tender story about a withdrawn 9-year-old who spends a fateful summer with two distant relatives. But the movie, adapted from a Claire Keegan story called Foster, doesn't rub your nose in the character's unhappiness. What brought me to tears more than once was the movie's unfashionable optimism — its insistence that goodness exists, and that simple acts of decency really can be life-changing.

The story is set in 1981, although given the remoteness of its rural Irish setting, it could easily be taking place decades earlier. The dialogue is subtitled, because the characters speak mostly Irish, a language we rarely hear in movies. The quiet girl of the title is named Cáit, and she's played with aching sensitivity by a gifted first-time actor named Catherine Clinch.

Cáit is the shyest and most neglected kid in her poor farming family. Her short-tempered mother has her hands full taking care of Cáit's siblings, and her father is a gambler, a philanderer and an all-around lout. At home and at school, Cáit does her best to stay under the radar. It's no wonder that the first time we see her, the camera has to pan down to find her hiding beneath tall blades of grass.

With too many mouths to feed and another baby on the way, it's decided that Cáit will spend the summer with relatives. Her mother's older cousin, Eibhlín, and her husband, Seán, live a three-hour drive away; they're played, wonderfully, by Carrie Crowley and Andrew Bennett. From the moment Eibhlín welcomes Cáit into their house, she lavishes the girl with kindness and attention. She engages her in conversation, involves her in household chores and responds in the most loving way when Caít wets the bed on her first night.

Seán is gruffer with Cáit at first, but he warms to her soon enough. There's a lovely little moment when, after angrily scolding her for wandering off by herself, Seán silently leaves a cookie on the table for her — an apology extended entirely without words. In their way, Eibhlín and Seán are as reserved as Cáit is, especially compared with some of their cruel, gossipy neighbors.

One of the most refreshing things about The Quiet Girl is that it doesn't treat silence as some problem that needs to be solved. When someone criticizes Cáit early on for being so quiet, Seán gently defends her, saying she "says as much as she has to say." And yet we see how Cáit gradually flourishes under her guardians' loving attention. Clinch's luminous performance shows us what it's like for a child to experience real, carefree happiness for the first time, whether it's Eibhlín offering Cáit a drink of crystalline water from the well near their house or Seán pressing a little pocket money into the girl's hands.

Seán and Eibhlín are clearly delighted by this temporary addition to their household, in part because it chases away some of the sorrow they've experienced in their own lives. The source of that sorrow isn't made clear right away, though you'll likely figure it out if you're paying close attention. When the truth does come out, it's treated with a gentle matter-of-factness that — much like the unfussy natural beauty of Kate McCullough's cinematography — deepens our sense of immersion in these characters' lives.

The Quiet Girl was written and directed by Colm Bairéad, an Irish filmmaker whose background is in documentaries. That may account in part for how exquisitely observed his first narrative feature is. Bairéad trusts the power of understatement, and that's a rare thing, given how prone so many films are to noise and over-explanation. Not many movies would focus on a character as unassuming as Cáit, but there's nothing small or insignificant about her story. Sometimes, it's the quietest movies that turn out to have the most to say.