Like most of the other Monach Islands, Shillay has a thin, sandy soil, which has been much troubled by erosion, and which was badly eroded by a huge wave in the 16th century.

Despite Ceann Iar's name, meaning "western headland", Shillay is the westernmost island of the group.

The red brick lighthouse was built in 1864 by David & Thomas Stevenson, and was in use until it was closed during the war in 1942 and not lit again in 1948 after hostilities ceased.

After the Braer disaster in 1993 in Shetland, a new light was recommended to mark the deep water route west of the Hebrides.

However it proved inadequate and in 2005 it was decided that it was cheaper to reuse the original lighthouse than to increase the height of the new light.