The lighthouse replaced an early one built on Cape Clear Island in 1818, partly motivated by the loss of an American sailing packet, Stephen Whitney, in thick fog during November 1847 on nearby West Calf Island causing the death of 92 of her 110 passengers and crew.

Various steps were taken to strengthen the tower, including fitting a casing around the bottom section up to the second floor and filling it with stone, and the surrounding rock smoothed over.

The replacement was constructed of stone, cast iron now being considered unsatisfactory – the whole of the nearby Calf tower above its strengthening casing had been carried away during a gale on 27 November 1881, although without loss of life.

A small steamship, the Ierne, was specially constructed for carrying the blocks out to the island, and Kavanagh personally set every stone, which weighed between 1.8 and 3.0 t (1.75 and 3 long tons).

The first floor of the original tower remains, on the highest part of the rock, having been left when it was demolished and converted into an oil store.

It produces a 0.14-second white flash every five seconds, with a nominal range of 27 nautical miles (50 kilometres) and a power of 2,500 kilocandelas.

In 1974 the explosive fog signal was replaced with an electric foghorn producing four blasts every minute at 300 hertz with a nominal range of 3.9 nautical miles (7.2 kilometres).

'Lone Rock')[5][6] or simply Fastnet (possibly from Old Norse Hvasstein-ey 'sharp-stone isle'), [citation needed] is a small clay-slate islet with quartz veins, and rises to about 30 metres (98 ft) above the low water mark and is separated from the much smaller Little Fastnet to the south by a 10-metre-wide (33 ft) channel.