The most westerly of the Heisker or Monach Isles, Shillay is today uninhabited and lies 8 miles (13 km) southwest of Aird an Runair, on the west coast of North Uist (Western Isles). The larger of the other islands being Ceann Iar, Ceann Ear, Shivinish and Stockay. The islands is separated from Ceann Iar by the Sound of Shillay.
In mediaeval times, monks living on the island maintained a light to guide sailors. Today, there are two lighthouses on this island, the older was built in 1864, but decommissioned in 1948. A new automated light was built in 1997 to delimit the Deep Water Route designated for the safe navigation of large tankers. The Natural Environment Research Council established a research-oriented nature reserve on the island in 1966.