From Wikipedia: http://en.wikipedia.org/wiki/Mussenden_Temple
Mussenden Temple is a small circular building located on cliffs near Castlerock in County Londonderry, high above the Atlantic Ocean on the north-western coast of Northern Ireland.
It was built in 1785 and forms part of the estate of Frederick Augustus Hervey, the 4th Earl of Bristol, Bishop of Derry. Built as a library and modelled on the Temple of Vesta in Italy, it is dedicated to the memory of Hervey's cousin Frideswide Mussenden.
Over the years the erosion of the cliff face at Downhill has brought Mussenden Temple ever closer to the edge, and in 1997 the National Trust carried out cliff stabilisation work to prevent the loss of the building.
The inscription around the building reads, "Suave, mari magno turbantibus aequora ventis e terra magnum alterius spectare laborem.." "Tis pleasant, safely to behold from shore/ The troubled sailor, and hear the tempests roar. The quotation is from Lucretius De Rerum Natura, 2.1-2
Now part of the National Trust property of Downhill Estate & Mussenden Temple, the grounds encompassing Mussenden Temple, and its manor house (Downhill Castle) are open to the public all year, from dawn to dusk.