The events of For Those in Peril take place in the westernmost portion of Wales, part of the United Kingdom on the island of Great Britain. Much of the action takes place at a lighthouse on the largest of a group of wave-washed rocks known as The Smalls, about 20 miles off the coast. The map below is an expansion of the highlighted area in the map above.
The action on shore takes place in an unspecified village in Mathry Parish, part of the county of Pembrokeshire. The village of Mathry can be seen toward the upper right of the map above.
The lighthouse in the opera was the first to be erected on The Smalls, in 1775, by a Liverpool instrument maker named Henry Whiteside, who appears as a character in the opera. His design, using nine oak pillars to support the lighthouse, allowed waves to pass through below the structure. The illustration above is from an 1859 book describing the history of the original lighthouse, which was replaced in 1861 by a more conventional stone tower.