Blanket peat behaves strangely. It looks solid underfoot, but it is mostly water held in a sponge of half-decomposed plant matter, the slow accumulation of thousands of Irish summers and winters. Pile concrete bases for industrial wind turbines on top of such a thing and dry it out for days on end, and you may discover, as the Derrybrien construction crew did, that a hillside has its limits. The mass began moving on 31 October 2003. It came to rest, initially, about two and a half kilometres downslope. Three weeks later the rains returned, the peat liquefied again, and the slide resumed - this time flowing into the Derrywee River, the Abhainn Da Loilioch, the river of two milkings.

Peat in moving water does not behave like sediment. It clogs, it smothers, it strips oxygen from the river. The slick travelled twenty kilometres before reaching Lough Cutra, the lake that supplied drinking water to the townland of Gort. Taps in Gort ran with a strange colour for days. Fish populations downstream collapsed in stretches where the peat settled. Two contractor companies were eventually convicted for pollution offences in a Galway court, though charges against the wind farm operator itself were dismissed. The legal story did not end there. In 2008 the European Court of Justice ruled against the Irish government - not for the slide itself, but for granting consent to build a wind farm on deep blanket peat without first conducting the environmental impact assessment that European law required.

Derrybrien became shorthand in Irish environmental circles for a specific kind of mistake: the assumption that renewable infrastructure, because it is renewable, does not need the same scrutiny as everything else. Blanket peat is one of the largest natural carbon stores in Ireland. Disturbing it releases carbon that took millennia to lock away. The wind farm did eventually come online, and turbines on the hillside still turn today, generating electricity for the grid. But the European court fines against Ireland kept accruing for years - daily penalties that ran into the millions before the case finally closed in 2019. Ireland paid, in the end, for cutting corners on a hill that could not bear the weight.

Stand on the slopes of Cashlaundrumlahan now and the scar is harder to see than it once was. Heather has grown back over much of the disturbed peat. The river runs clear most days. The turbines, slim white columns against the grey-green of the Slieve Aughty Mountains, look as innocent as wind farms anywhere. But the geography of caution shifted that Halloween. Every subsequent Irish wind farm proposed on peatland faced a different conversation - one informed by what happened when a hill near Derrybrien decided, after weeks of drought, that it had had enough.