LAUGHARNE, Wales — Down the footpath from his writing shed, along the curve of the water and up the hill, you see what the poet Dylan Thomas once saw: tall birds on the “heron priested shore,” a “sea wet church the size of a snail” atop the ridge, the castle ruin to your left still “brown as owls.”

“Poem in October,” in which Thomas reflects on his 30th birthday, unfolds verse after verse as you walk through the landscape that made him, and that he remade in turn, culminating with a final cliff-top exclamation:

“O may my heart’s truth

still be sung

on this high hill in a year’s turning.”

Thomas died young, at 39, after boasting that he had downed 18 straight whiskeys (“I believe that’s the record”) in New York in 1953. On Monday, he would have turned 100. His small country, long ill at ease with its hard-living, hard-loving son who wrote in English, not in Welsh, and caricatured his roots as much as he claimed them, is celebrating perhaps its greatest poet.

Thomas has been called the James Joyce of Wales and compared to his own hero, John Keats. He wrote some of the most recognizable verse of the 20th century: “Do not go gentle into that good night/Rage, rage against the dying of the light.”

Gillian Clarke, the national poet of Wales, who grew up in this part of western Wales, traces her own poetic awakening to the day she first heard Thomas read on the BBC, his voice summoning her 15-year-old self to “the sloeblack, slow, black, crowblack fishboat-bobbing sea” that she too knew so well. “He takes from and gives back to the landscape and the language, until the place speaks the poetry as much as the poetry voices the landscape,” Ms. Clarke said.