The island is formed by stabilising shingle, gravel and stones overlain by a thin layer of soil.
The ASSI is also an SPA[1] (Special Protection Area) because it regularly supports internationally important numbers of light-bellied brent geese in winter.
Swan Island, which was classified as an SPA in its own right in 1992, qualifies because, in summer, it supports nationally important breeding populations of rosate and common terns.
[2][3] The nationally rare plant bur chervil can also be found in significant abundance on Swan Island.
This article related to the geography of County Antrim, Northern Ireland is a stub.