Could this be the island you are referring to ?
The smallest of the Forth islands is perhaps both its most unheralded and most important. One of the four inches (Gaelic: island) of the Forth, Inchgarvie, which sits almost mid-firth between North and South Queensferry in Fife, made possible the construction of the Forth Rail Bridge (completed in 1890) by providing a base for its central cantilever at the estuary's narrowest point.
Like all the other islands, Inchgarvie was occupied by the army during the two world wars, and the remains of military installations to defend the rail bridge and the nearby Rosyth naval base are clearly visible from land or a passing boat. (The island is not accessible to the public.)
Much earlier there was a castle on Inchgarvie, which was attacked and mostly demolished by Oliver Cromwell. Later it was used as a prison and quarantine station. Today it is home to a range of seabirds, including fulmars, cormorants and puffins.
Found this information here:
http://www.tartan-umbrella.com/features_copy.asp?ID=Firth%20of%20Forth