Glash Signal Tower, Nakil Or Surgeview, Co. Mayo

That combination is precisely what makes it worth pausing over: a defensive structure from the age of muskets and telescopes, dressed in the borrowed vocabulary of the fifteenth century. The square plan, the sloped or "battered" base that thickens the walls towards the ground, and the corbelled bartizans (small overhanging turrets projecting from the corners) were not decorative whims. They belonged to a standard prototype used up and down the west coast, from Donegal to Cork, and Glash is one of the few surviving examples built to that exact template.

The tower dates from 1804 to 1806, a direct response to a very real scare. In 1798 the French general Jean Joseph Amable Humbert had landed troops at Kilcummin Head in support of the United Irishmen, proving that an Atlantic landing was no theoretical exercise. As the Napoleonic Wars ground on, the British administration ordered a chain of signal towers built along the entire western seaboard, each designed to the same compact formula so that a fire could be lit or a flag hoisted at one tower and relayed, station to station, at speed if an enemy fleet appeared offshore. County Mayo alone had eight such towers. The system's Achilles heel was the weather; the misty Atlantic climate frequently swallowed the visual signals before they could travel far, undermining the very purpose of the network. Once the naval victory at Trafalgar in 1805 removed the immediate threat of invasion, the towers were abandoned almost as quickly as they had been built, left to weather without any further military purpose. By 1916 Glash was already recorded as a ruin, and the roof, window dressings, and rendered wall finish that once covered the rubble stone have since disappeared entirely.

Of Mayo's original eight towers, Glash is now the only one to survive in anything like a complete state; a neighbouring example, Shivel Head, partially collapsed in a storm in 1989. What remains at Glash is essentially a shell, two stages tall, with paired square-headed window openings on each floor and a fireplace with flanking niches still traceable inside, centred on the ruin of a chimneystack on the east wall. The grounds around it are unkempt rather than maintained, which suits a structure that was always meant to be functional and remote rather than comfortable, a lookout post built for a war that never quite arrived on this shore.