The Marin County Starfleet Yards are a number of Federation shipyards, located in the Sol sector, with surface facilities presumably located in Marin County, in the San Francisco Bay Area, on Earth.
In an unknown, alternate future timeframe, the USS Pasteur was built at the Marin County Starfleet Yards by the Skywalker Division. (TNG: "All Good Things...")
This is a tribute to Industrial Light & Magic, a special effects company based at Skywalker Ranch in Marin County, California, and founded by George Lucas. ILM created many visual effects shots used in Star Trek: The Next Generation.