Ordnance Survey maps show that the north-eastern pier in that group of four is planted directly on the island, but below the high-water line. The other three piers are founded on a shelf of rock that slopes away from the island towards the south.
The lighthouse under the bridge there (also visible in Google's imagery) marks the western end of the island.
The "HWMOST" and "LWMOST" outlines are the high- and low-water marks of ordinary spring tides, as at 1960 and 1954 respectively.
(reproduced from OS Plan NT 1379, revised 1960 and published 1961, made available by the National Library of Scotland at
https://maps.nls.uk/view/130077368)
The island was used for offices and the accommodation of workers during the construction of the bridge, and the OS map even shows a footbridge linking the high ground on the island to the bridge pier.
The lighthouse is of some historic interest (if you'll forgive the diversion from the topic) in that it's built upon the pier that was constructed for the
original Forth Bridge - Thomas Bouch's suspension bridge, further work on which was cancelled following the Tay Bridge disaster.