Glen Signal Tower, Sliabh Bhíofáin Agus Gharbhrois, Co. Donegal

The main entrance sat below a machicolation, a projecting stone gallery through which defenders could drop objects on anyone attempting to force their way in, a small but telling sign of just how seriously this modest building's occupants took the threat of attack. Two more machicolations project from the north-east and south-east corners. The tower's flat roof has long since collapsed behind its parapet, and one corner of that parapet has partially fallen away, yet enough survives, including the cut stone surrounds around its windows, to give a clear sense of what a signal tower looked like when new.

Glen Signal Tower belongs to a network of more than eighty such structures built along Ireland's west, south, south-east and north-west coasts between 1804 and 1806, a direct response to French invasion attempts in the 1790s and, in particular, the French landing at Killala in County Mayo in 1798. Built to a broadly standard design, the towers relied on a system of flags and balls hoisted on a mast roughly fifty feet high on the seaward side; a message raised at one tower could be read and relayed by the next, carrying warning of enemy ships along the coast station by station. This meant the towers needed unobstructed sightlines to their neighbours, which is why they invariably occupy high, exposed ground, in this case looking towards Malin Beg to the south and Dawros Head to the north-east. Work at Glen Head began in 1804 and finished by September 1806, overseen by Major-General Sir Charles Ross, with the engineering possibly the work of Sir William Smith, who is associated with several of the Donegal towers, at a construction cost of somewhere between six hundred and nine hundred pounds. The urgency behind the whole programme evaporated remarkably quickly: after Nelson's victory at Trafalgar in 1805 removed any realistic prospect of invasion, the towers were judged surplus to requirement, and by 1809 the network had already been abandoned.

What makes this particular tower worth seeking out is its condition relative to others of its kind. Many signal towers around the Irish coast have lost their finely cut stone door and window surrounds to later scavenging, but Glen Head's remain largely intact, along with traces of the lime roughcast render that would once have covered the rubble stone walls. Its cliff top position gives views west and north-west over the open Atlantic and south towards Skelpoonagh Bay and Glen Bay, the same commanding outlook that made the site useful to its builders in the first place.