Carrownrush Tower, Carrownrush, Co. Sligo

Except this one has nothing to do with the Gaelic lordships that raised those fortified residences centuries earlier. It was built around 1805, during the Napoleonic wars, as one link in a chain of watch-and-warning posts strung around the Irish coast to spot a French invasion fleet before it could land.

The design borrowed deliberately from an older tradition. Tower houses, the small fortified residences built by Irish and Anglo-Irish landowners from the late medieval period onward, had long relied on projecting corner turrets called bartizans for defence and observation, and on corbelling, stone courses stepped outward course by course to carry weight without visible support, to create these overhanging features cheaply and durably. The engineers of the signal tower network reached for the same vocabulary three hundred years later, not out of sentiment but because it worked: a small garrison needed a defensible, weatherproof lookout, and the tower house had already solved that problem. At Carrownrush the builders even added a corbelled garderobe, a projecting latrine, to the centre of the north elevation, a detail lifted straight from castle architecture. The structure saw further work at parapet level around 1850, decades after the invasion scare that prompted its construction had passed, suggesting it found some continued use even once its original purpose had lapsed.

What survives today is the shell: rubble stone walls rising two storeys, square-headed window openings on the north, east and west faces framed in cut ashlar, and ashlar copings still running along the parapets. The windows themselves are gone, the entrance, thought to have faced east, is no longer traceable, and both internal floors have disappeared, leaving the interior open to the sky in effect. The tower sits on a promontory looking out over Sligo Bay, a position chosen for exactly the reason its Napoleonic-era garrison would have wanted: a clear, uninterrupted view out to sea, from which any approaching sail would have been visible long before it reached the shore.