Inishtrahull Lighthouse, Inishtrahull, Co. Donegal

What catches the eye, once you know to look for it, is not just the tower itself but everything around it: a hexagonal fog signal building from 1905, a helicopter pad, a pair of semi-detached former keepers' houses, and, at the far end of the island, the stump of an older lighthouse entirely, left standing as a kind of ruin twin to the working structure at the western end.

The current lighthouse was built between 1956 and 1958 to designs by A. D. H. Martin, who would go on to become Engineer-in-Chief at the Commissioners of Irish Lights, and it holds the distinction of being the first lighthouse the Commissioners built in Ireland during the twentieth century. It replaced an 1812 to 1813 structure whose remains still sit at the opposite end of Inishtrahull, a rare instance of an island carrying two generations of lighthouse rather than one absorbing or effacing the other. The original light threw a beam some twenty nautical miles with an intensity of 1.75 million candelas; automation in the 1980s brought a new light of lower intensity, 760,000 candelas, but greater range, at 25 nautical miles. The fog signal building to the north-east predates the current tower by half a century: it was constructed in 1905 for the Royal Navy, to assist naval shipping using Lough Swilly, and its diaphone signal, a foghorn mechanism that produces a distinctive low, two-toned blast, was later mounted directly above the lantern of the new lighthouse itself. Older still is the site's role as a Lloyd's Signal Station, where the arrival of transatlantic shipping from North America was relayed by semaphore to stations on Malin Head, and from there to Lloyd's of London, whose underwriters depended on exactly this kind of early warning for the insurance of transatlantic voyages.

The lighthouse, its ancillary buildings, the keepers' houses, and the ruined 1812 structure together form a compact record of how this stretch of coast was watched, signalled from, and warned around for the best part of two centuries, each layer, naval, commercial, and maritime-safety, still legible in the cluster of structures at either end of the island.