This is the historic Tide Mill Building, on Harbor Road in the Southport Historic District of Southport, CT section in Fairfield, CT.
The building was originally a tide mill, which is a water mill that is driven by the rise and fall of the tide. The Tide Mill building was also a tavern prior to its current mixed-use (office/residential) status. The most notable tenant is the office of J.P. Franzen Associates Architects, P.C. The land it sits on was once an island, until it was connected by the two bridges. According to the owner of the building and the parking lot across the street, "the area was once known as Herdsman's Island. The colonists would bring their sheep there to protect them from the wolves," he said. The building is located on the Mill River, which flows into Long Island Sound.
The Southport Historic Distric an area that was listed on the National Register of Historic Places in 1971.
The following links provide additional details and photographs of the structure, as well as information on the tenants: