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.
There are several small islets around Shillay, including Eilean Siorraidh, Odarum (to the north) and Raisgeir.
Shillay is most notable for being the location of a disused lighthouse. This was originally a fire, which was maintained by the monks who lived at Clach na Bleide on Ceann Ear. The red brick lighthouse was built in 1864 by David & Thomas Stevenson, and was in use until 1942. A newer light was installed in 1997.
A stone from the lighthouse has been removed into the church wall at Paible; a keeper had carved into it "Eternity Oh Eternity". Interestingly, just to the south of Shillay, there is an Eilean Siorraidh ("Island of Eternity"; formerly Eilean Siorruidh) - whether this carving is a reference to this or the solitude of the island is unrecorded.